<!-- doc/src/sgml/monitoring.sgml -->

<chapter id="monitoring">
 <title>Monitoring Database Activity</title>

 <indexterm zone="monitoring">
  <primary>monitoring</primary>
  <secondary>database activity</secondary>
 </indexterm>

 <indexterm zone="monitoring">
  <primary>database activity</primary>
  <secondary>monitoring</secondary>
 </indexterm>

 <para>
  A database administrator frequently wonders, <quote>What is the system
  doing right now?</quote>
  This chapter discusses how to find that out.
 </para>

  <para>
   Several tools are available for monitoring database activity and
   analyzing performance.  Most of this chapter is devoted to describing
   <productname>PostgreSQL</productname>'s statistics collector,
   but one should not neglect regular Unix monitoring programs such as
   <command>ps</command>, <command>top</command>, <command>iostat</command>, and <command>vmstat</command>.
   Also, once one has identified a
   poorly-performing query, further investigation might be needed using
   <productname>PostgreSQL</productname>'s <xref linkend="sql-explain"/> command.
   <xref linkend="using-explain"/> discusses <command>EXPLAIN</command>
   and other methods for understanding the behavior of an individual
   query.
  </para>

 <sect1 id="monitoring-ps">
  <title>Standard Unix Tools</title>

  <indexterm zone="monitoring-ps">
   <primary>ps</primary>
   <secondary>to monitor activity</secondary>
  </indexterm>

  <para>
   On most Unix platforms, <productname>PostgreSQL</productname> modifies its
   command title as reported by <command>ps</command>, so that individual server
   processes can readily be identified.  A sample display is

<screen>
$ ps auxww | grep ^postgres
postgres  15551  0.0  0.1  57536  7132 pts/0    S    18:02   0:00 postgres -i
postgres  15554  0.0  0.0  57536  1184 ?        Ss   18:02   0:00 postgres: background writer
postgres  15555  0.0  0.0  57536   916 ?        Ss   18:02   0:00 postgres: checkpointer
postgres  15556  0.0  0.0  57536   916 ?        Ss   18:02   0:00 postgres: walwriter
postgres  15557  0.0  0.0  58504  2244 ?        Ss   18:02   0:00 postgres: autovacuum launcher
postgres  15558  0.0  0.0  17512  1068 ?        Ss   18:02   0:00 postgres: stats collector
postgres  15582  0.0  0.0  58772  3080 ?        Ss   18:04   0:00 postgres: joe runbug 127.0.0.1 idle
postgres  15606  0.0  0.0  58772  3052 ?        Ss   18:07   0:00 postgres: tgl regression [local] SELECT waiting
postgres  15610  0.0  0.0  58772  3056 ?        Ss   18:07   0:00 postgres: tgl regression [local] idle in transaction
</screen>

   (The appropriate invocation of <command>ps</command> varies across different
   platforms, as do the details of what is shown.  This example is from a
   recent Linux system.)  The first process listed here is the
   master server process.  The command arguments
   shown for it are the same ones used when it was launched.  The next five
   processes are background worker processes automatically launched by the
   master process.  (The <quote>stats collector</quote> process will not be present
   if you have set the system not to start the statistics collector; likewise
   the <quote>autovacuum launcher</quote> process can be disabled.)
   Each of the remaining
   processes is a server process handling one client connection.  Each such
   process sets its command line display in the form

<screen>
postgres: <replaceable>user</replaceable> <replaceable>database</replaceable> <replaceable>host</replaceable> <replaceable>activity</replaceable>
</screen>

  The user, database, and (client) host items remain the same for
  the life of the client connection, but the activity indicator changes.
  The activity can be <literal>idle</literal> (i.e., waiting for a client command),
  <literal>idle in transaction</literal> (waiting for client inside a <command>BEGIN</command> block),
  or a command type name such as <literal>SELECT</literal>.  Also,
  <literal>waiting</literal> is appended if the server process is presently waiting
  on a lock held by another session.  In the above example we can infer
  that process 15606 is waiting for process 15610 to complete its transaction
  and thereby release some lock.  (Process 15610 must be the blocker, because
  there is no other active session.  In more complicated cases it would be
  necessary to look into the
  <link linkend="view-pg-locks"><structname>pg_locks</structname></link>
  system view to determine who is blocking whom.)
  </para>

  <para>
   If <xref linkend="guc-cluster-name"/> has been configured the
   cluster name will also be shown in <command>ps</command> output:
<screen>
$ psql -c 'SHOW cluster_name'
 cluster_name
--------------
 server1
(1 row)

$ ps aux|grep server1
postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: server1: background writer
...
</screen>
  </para>

  <para>
   If you have turned off <xref linkend="guc-update-process-title"/> then the
   activity indicator is not updated; the process title is set only once
   when a new process is launched.  On some platforms this saves a measurable
   amount of per-command overhead;  on others it's insignificant.
  </para>

  <tip>
  <para>
  <productname>Solaris</productname> requires special handling. You must
  use <command>/usr/ucb/ps</command>, rather than
  <command>/bin/ps</command>. You also must use two <option>w</option>
  flags, not just one. In addition, your original invocation of the
  <command>postgres</command> command must have a shorter
  <command>ps</command> status display than that provided by each
  server process.  If you fail to do all three things, the <command>ps</command>
  output for each server process will be the original <command>postgres</command>
  command line.
  </para>
  </tip>
 </sect1>

 <sect1 id="monitoring-stats">
  <title>The Statistics Collector</title>

  <indexterm zone="monitoring-stats">
   <primary>statistics</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname>'s <firstterm>statistics collector</firstterm>
   is a subsystem that supports collection and reporting of information about
   server activity.  Presently, the collector can count accesses to tables
   and indexes in both disk-block and individual-row terms.  It also tracks
   the total number of rows in each table, and information about vacuum and
   analyze actions for each table.  It can also count calls to user-defined
   functions and the total time spent in each one.
  </para>

  <para>
   <productname>PostgreSQL</productname> also supports reporting dynamic
   information about exactly what is going on in the system right now, such as
   the exact command currently being executed by other server processes, and
   which other connections exist in the system.  This facility is independent
   of the collector process.
  </para>

 <sect2 id="monitoring-stats-setup">
  <title>Statistics Collection Configuration</title>

  <para>
   Since collection of statistics adds some overhead to query execution,
   the system can be configured to collect or not collect information.
   This is controlled by configuration parameters that are normally set in
   <filename>postgresql.conf</filename>.  (See <xref linkend="runtime-config"/> for
   details about setting configuration parameters.)
  </para>

  <para>
   The parameter <xref linkend="guc-track-activities"/> enables monitoring
   of the current command being executed by any server process.
  </para>

  <para>
   The parameter <xref linkend="guc-track-counts"/> controls whether
   statistics are collected about table and index accesses.
  </para>

  <para>
   The parameter <xref linkend="guc-track-functions"/> enables tracking of
   usage of user-defined functions.
  </para>

  <para>
   The parameter <xref linkend="guc-track-io-timing"/> enables monitoring
   of block read and write times.
  </para>

  <para>
   Normally these parameters are set in <filename>postgresql.conf</filename> so
   that they apply to all server processes, but it is possible to turn
   them on or off in individual sessions using the <xref
   linkend="sql-set"/> command. (To prevent
   ordinary users from hiding their activity from the administrator,
   only superusers are allowed to change these parameters with
   <command>SET</command>.)
  </para>

  <para>
   The statistics collector transmits the collected information to other
   <productname>PostgreSQL</productname> processes through temporary files.
   These files are stored in the directory named by the
   <xref linkend="guc-stats-temp-directory"/> parameter,
   <filename>pg_stat_tmp</filename> by default.
   For better performance, <varname>stats_temp_directory</varname> can be
   pointed at a RAM-based file system, decreasing physical I/O requirements.
   When the server shuts down cleanly, a permanent copy of the statistics
   data is stored in the <filename>pg_stat</filename> subdirectory, so that
   statistics can be retained across server restarts.  When recovery is
   performed at server start (e.g. after immediate shutdown, server crash,
   and point-in-time recovery), all statistics counters are reset.
  </para>

 </sect2>

 <sect2 id="monitoring-stats-views">
  <title>Viewing Statistics</title>

  <para>
   Several predefined views, listed in <xref
   linkend="monitoring-stats-dynamic-views-table"/>, are available to show
   the current state of the system. There are also several other
   views, listed in <xref
   linkend="monitoring-stats-views-table"/>, available to show the results
   of statistics collection.  Alternatively, one can
   build custom views using the underlying statistics functions, as discussed
   in <xref linkend="monitoring-stats-functions"/>.
  </para>

  <para>
   When using the statistics to monitor collected data, it is important
   to realize that the information does not update instantaneously.
   Each individual server process transmits new statistical counts to
   the collector just before going idle; so a query or transaction still in
   progress does not affect the displayed totals.  Also, the collector itself
   emits a new report at most once per <varname>PGSTAT_STAT_INTERVAL</varname>
   milliseconds (500 ms unless altered while building the server).  So the
   displayed information lags behind actual activity.  However, current-query
   information collected by <varname>track_activities</varname> is
   always up-to-date.
  </para>

  <para>
   Another important point is that when a server process is asked to display
   any of these statistics, it first fetches the most recent report emitted by
   the collector process and then continues to use this snapshot for all
   statistical views and functions until the end of its current transaction.
   So the statistics will show static information as long as you continue the
   current transaction.  Similarly, information about the current queries of
   all sessions is collected when any such information is first requested
   within a transaction, and the same information will be displayed throughout
   the transaction.
   This is a feature, not a bug, because it allows you to perform several
   queries on the statistics and correlate the results without worrying that
   the numbers are changing underneath you.  But if you want to see new
   results with each query, be sure to do the queries outside any transaction
   block.  Alternatively, you can invoke
   <function>pg_stat_clear_snapshot</function>(), which will discard the
   current transaction's statistics snapshot (if any).  The next use of
   statistical information will cause a new snapshot to be fetched.
  </para>

  <para>
   A transaction can also see its own statistics (as yet untransmitted to the
   collector) in the views <structname>pg_stat_xact_all_tables</structname>,
   <structname>pg_stat_xact_sys_tables</structname>,
   <structname>pg_stat_xact_user_tables</structname>, and
   <structname>pg_stat_xact_user_functions</structname>.  These numbers do not act as
   stated above; instead they update continuously throughout the transaction.
  </para>

  <table id="monitoring-stats-dynamic-views-table">
   <title>Dynamic Statistics Views</title>

   <tgroup cols="2">
    <thead>
     <row>
      <entry>View Name</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry>
       <structname>pg_stat_activity</structname>
       <indexterm><primary>pg_stat_activity</primary></indexterm>
      </entry>
      <entry>
       One row per server process, showing information related to
       the current activity of that process, such as state and current query.
       See <xref linkend="pg-stat-activity-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_replication</structname><indexterm><primary>pg_stat_replication</primary></indexterm></entry>
      <entry>One row per WAL sender process, showing statistics about
       replication to that sender's connected standby server.
       See <xref linkend="pg-stat-replication-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_wal_receiver</structname><indexterm><primary>pg_stat_wal_receiver</primary></indexterm></entry>
      <entry>Only one row, showing statistics about the WAL receiver from
       that receiver's connected server.
       See <xref linkend="pg-stat-wal-receiver-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_subscription</structname><indexterm><primary>pg_stat_subscription</primary></indexterm></entry>
      <entry>At least one row per subscription, showing information about
       the subscription workers.
       See <xref linkend="pg-stat-subscription"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_ssl</structname><indexterm><primary>pg_stat_ssl</primary></indexterm></entry>
      <entry>One row per connection (regular and replication), showing information about
       SSL used on this connection.
       See <xref linkend="pg-stat-ssl-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_progress_vacuum</structname><indexterm><primary>pg_stat_progress_vacuum</primary></indexterm></entry>
      <entry>One row for each backend (including autovacuum worker processes) running
       <command>VACUUM</command>, showing current progress.
       See <xref linkend='vacuum-progress-reporting'/>.
      </entry>
     </row>

    </tbody>
   </tgroup>
  </table>

  <table id="monitoring-stats-views-table">
   <title>Collected Statistics Views</title>

   <tgroup cols="2">
    <thead>
     <row>
      <entry>View Name</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry><structname>pg_stat_archiver</structname><indexterm><primary>pg_stat_archiver</primary></indexterm></entry>
      <entry>One row only, showing statistics about the
       WAL archiver process's activity. See
       <xref linkend="pg-stat-archiver-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_bgwriter</structname><indexterm><primary>pg_stat_bgwriter</primary></indexterm></entry>
      <entry>One row only, showing statistics about the
       background writer process's activity. See
       <xref linkend="pg-stat-bgwriter-view"/> for details.
     </entry>
     </row>

     <row>
      <entry><structname>pg_stat_database</structname><indexterm><primary>pg_stat_database</primary></indexterm></entry>
      <entry>One row per database, showing database-wide statistics. See
       <xref linkend="pg-stat-database-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_database_conflicts</structname><indexterm><primary>pg_stat_database_conflicts</primary></indexterm></entry>
      <entry>
       One row per database, showing database-wide statistics about
       query cancels due to conflict with recovery on standby servers.
       See <xref linkend="pg-stat-database-conflicts-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_all_tables</structname><indexterm><primary>pg_stat_all_tables</primary></indexterm></entry>
      <entry>
       One row for each table in the current database, showing statistics
       about accesses to that specific table.
       See <xref linkend="pg-stat-all-tables-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_sys_tables</structname><indexterm><primary>pg_stat_sys_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_all_tables</structname>, except that only
      system tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_user_tables</structname><indexterm><primary>pg_stat_user_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_all_tables</structname>, except that only user
      tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_xact_all_tables</structname><indexterm><primary>pg_stat_xact_all_tables</primary></indexterm></entry>
      <entry>Similar to <structname>pg_stat_all_tables</structname>, but counts actions
      taken so far within the current transaction (which are <emphasis>not</emphasis>
      yet included in <structname>pg_stat_all_tables</structname> and related views).
      The columns for numbers of live and dead rows and vacuum and
      analyze actions are not present in this view.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_xact_sys_tables</structname><indexterm><primary>pg_stat_xact_sys_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_xact_all_tables</structname>, except that only
      system tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_xact_user_tables</structname><indexterm><primary>pg_stat_xact_user_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_xact_all_tables</structname>, except that only
      user tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_all_indexes</structname><indexterm><primary>pg_stat_all_indexes</primary></indexterm></entry>
      <entry>
       One row for each index in the current database, showing statistics
       about accesses to that specific index.
       See <xref linkend="pg-stat-all-indexes-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_sys_indexes</structname><indexterm><primary>pg_stat_sys_indexes</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_all_indexes</structname>, except that only
      indexes on system tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_user_indexes</structname><indexterm><primary>pg_stat_user_indexes</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_all_indexes</structname>, except that only
      indexes on user tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_statio_all_tables</structname><indexterm><primary>pg_statio_all_tables</primary></indexterm></entry>
      <entry>
       One row for each table in the current database, showing statistics
       about I/O on that specific table.
       See <xref linkend="pg-statio-all-tables-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_statio_sys_tables</structname><indexterm><primary>pg_statio_sys_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_tables</structname>, except that only
      system tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_statio_user_tables</structname><indexterm><primary>pg_statio_user_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_tables</structname>, except that only
      user tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_statio_all_indexes</structname><indexterm><primary>pg_statio_all_indexes</primary></indexterm></entry>
      <entry>
       One row for each index in the current database,
       showing statistics about I/O on that specific index.
       See <xref linkend="pg-statio-all-indexes-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_statio_sys_indexes</structname><indexterm><primary>pg_statio_sys_indexes</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_indexes</structname>, except that only
      indexes on system tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_statio_user_indexes</structname><indexterm><primary>pg_statio_user_indexes</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_indexes</structname>, except that only
      indexes on user tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_statio_all_sequences</structname><indexterm><primary>pg_statio_all_sequences</primary></indexterm></entry>
     <entry>
       One row for each sequence in the current database,
       showing statistics about I/O on that specific sequence.
       See <xref linkend="pg-statio-all-sequences-view"/> for details.
     </entry>
     </row>

     <row>
      <entry><structname>pg_statio_sys_sequences</structname><indexterm><primary>pg_statio_sys_sequences</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_sequences</structname>, except that only
      system sequences are shown.  (Presently, no system sequences are defined,
      so this view is always empty.)</entry>
     </row>

     <row>
      <entry><structname>pg_statio_user_sequences</structname><indexterm><primary>pg_statio_user_sequences</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_sequences</structname>, except that only
      user sequences are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_user_functions</structname><indexterm><primary>pg_stat_user_functions</primary></indexterm></entry>
      <entry>
       One row for each tracked function, showing statistics
       about executions of that function. See
       <xref linkend="pg-stat-user-functions-view"/> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_xact_user_functions</structname><indexterm><primary>pg_stat_xact_user_functions</primary></indexterm></entry>
      <entry>Similar to <structname>pg_stat_user_functions</structname>, but counts only
      calls during the current transaction (which are <emphasis>not</emphasis>
      yet included in <structname>pg_stat_user_functions</structname>).</entry>
     </row>

    </tbody>
   </tgroup>
  </table>

  <para>
   The per-index statistics are particularly useful to determine which
   indexes are being used and how effective they are.
  </para>

  <para>
   The <structname>pg_statio_</structname> views are primarily useful to
   determine the effectiveness of the buffer cache.  When the number
   of actual disk reads is much smaller than the number of buffer
   hits, then the cache is satisfying most read requests without
   invoking a kernel call. However, these statistics do not give the
   entire story: due to the way in which <productname>PostgreSQL</productname>
   handles disk I/O, data that is not in the
   <productname>PostgreSQL</productname> buffer cache might still reside in the
   kernel's I/O cache, and might therefore still be fetched without
   requiring a physical read. Users interested in obtaining more
   detailed information on <productname>PostgreSQL</productname> I/O behavior are
   advised to use the <productname>PostgreSQL</productname> statistics collector
   in combination with operating system utilities that allow insight
   into the kernel's handling of I/O.
  </para>


  <table id="pg-stat-activity-view" xreflabel="pg_stat_activity">
   <title><structname>pg_stat_activity</structname> View</title>

   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>datid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of the database this backend is connected to</entry>
    </row>
    <row>
     <entry><structfield>datname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the database this backend is connected to</entry>
    </row>
    <row>
     <entry><structfield>pid</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>Process ID of this backend</entry>
    </row>
    <row>
     <entry><structfield>usesysid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of the user logged into this backend</entry>
    </row>
    <row>
     <entry><structfield>usename</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the user logged into this backend</entry>
    </row>
    <row>
     <entry><structfield>application_name</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Name of the application that is connected
      to this backend</entry>
    </row>
    <row>
     <entry><structfield>client_addr</structfield></entry>
     <entry><type>inet</type></entry>
     <entry>IP address of the client connected to this backend.
      If this field is null, it indicates either that the client is
      connected via a Unix socket on the server machine or that this is an
      internal process such as autovacuum.
     </entry>
    </row>
    <row>
     <entry><structfield>client_hostname</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Host name of the connected client, as reported by a
      reverse DNS lookup of <structfield>client_addr</structfield>. This field will
      only be non-null for IP connections, and only when <xref
      linkend="guc-log-hostname"/> is enabled.
     </entry>
    </row>
    <row>
     <entry><structfield>client_port</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>TCP port number that the client is using for communication
      with this backend, or <literal>-1</literal> if a Unix socket is used
     </entry>
    </row>
    <row>
     <entry><structfield>backend_start</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Time when this process was started.  For client backends,
      this is the time the client connected to the server.
     </entry>
    </row>
    <row>
     <entry><structfield>xact_start</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Time when this process' current transaction was started, or null
      if no transaction is active. If the current
      query is the first of its transaction, this column is equal to the
      <structfield>query_start</structfield> column.
     </entry>
    </row>
    <row>
     <entry><structfield>query_start</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Time when the currently active query was started, or if
      <structfield>state</structfield> is not <literal>active</literal>, when the last query
      was started
     </entry>
    </row>
    <row>
     <entry><structfield>state_change</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Time when the <structfield>state</structfield> was last changed</entry>
    </row>
     <row>
      <entry><structfield>wait_event_type</structfield></entry>
      <entry><type>text</type></entry>
      <entry>The type of event for which the backend is waiting, if any;
       otherwise NULL. Possible values are:
       <itemizedlist>
        <listitem>
         <para>
          <literal>LWLock</literal>: The backend is waiting for a lightweight lock.
          Each such lock protects a particular data structure in shared memory.
          <literal>wait_event</literal> will contain a name identifying the purpose
          of the lightweight lock.  (Some locks have specific names; others
          are part of a group of locks each with a similar purpose.)
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>Lock</literal>: The backend is waiting for a heavyweight lock.
          Heavyweight locks, also known as lock manager locks or simply locks,
          primarily protect SQL-visible objects such as tables.  However,
          they are also used to ensure mutual exclusion for certain internal
          operations such as relation extension.  <literal>wait_event</literal> will
          identify the type of lock awaited.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>BufferPin</literal>: The server process is waiting to access to
          a data buffer during a period when no other process can be
          examining that buffer.  Buffer pin waits can be protracted if
          another process holds an open cursor which last read data from the
          buffer in question.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>Activity</literal>: The server process is idle.  This is used by
          system processes waiting for activity in their main processing loop.
          <literal>wait_event</literal> will identify the specific wait point.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>Extension</literal>: The server process is waiting for activity
          in an extension module.  This category is useful for modules to
          track custom waiting points.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>Client</literal>: The server process is waiting for some activity
          on a socket from user applications, and that the server expects
          something to happen that is independent from its internal processes.
          <literal>wait_event</literal> will identify the specific wait point.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>IPC</literal>: The server process is waiting for some activity
          from another process in the server.  <literal>wait_event</literal> will
          identify the specific wait point.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>Timeout</literal>: The server process is waiting for a timeout
          to expire.  <literal>wait_event</literal> will identify the specific wait
          point.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>IO</literal>: The server process is waiting for a IO to complete.
          <literal>wait_event</literal> will identify the specific wait point.
         </para>
        </listitem>
       </itemizedlist>
      </entry>
     </row>
    <row>
     <entry><structfield>wait_event</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Wait event name if backend is currently waiting, otherwise NULL.
     See <xref linkend="wait-event-table"/> for details.
     </entry>
    </row>
    <row>
     <entry><structfield>state</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Current overall state of this backend.
       Possible values are:
       <itemizedlist>
         <listitem>
          <para>
           <literal>active</literal>: The backend is executing a query.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>idle</literal>: The backend is waiting for a new client command.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>idle in transaction</literal>: The backend is in a transaction,
           but is not currently executing a query.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>idle in transaction (aborted)</literal>: This state is similar to
           <literal>idle in transaction</literal>, except one of the statements in
           the transaction caused an error.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>fastpath function call</literal>: The backend is executing a
           fast-path function.
          </para>
         </listitem>
         <listitem>
           <para>
           <literal>disabled</literal>: This state is reported if <xref
           linkend="guc-track-activities"/> is disabled in this backend.
          </para>
         </listitem>
       </itemizedlist>
     </entry>
    </row>
    <row>
     <entry><structfield>backend_xid</structfield></entry>
     <entry><type>xid</type></entry>
     <entry>Top-level transaction identifier of this backend, if any.</entry>
    </row>
    <row>
     <entry><structfield>backend_xmin</structfield></entry>
     <entry><type>xid</type></entry>
     <entry>The current backend's <literal>xmin</literal> horizon.</entry>
    </row>
    <row>
     <entry><structfield>query</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Text of this backend's most recent query. If
      <structfield>state</structfield> is <literal>active</literal> this field shows the
      currently executing query. In all other states, it shows the last query
      that was executed. By default the query text is truncated at 1024
      characters; this value can be changed via the parameter
      <xref linkend="guc-track-activity-query-size"/>.
     </entry>
    </row>
    <row>
     <entry><structfield>backend_type</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Type of current backend. Possible types are
      <literal>autovacuum launcher</literal>, <literal>autovacuum worker</literal>,
      <literal>background worker</literal>, <literal>background writer</literal>,
      <literal>client backend</literal>, <literal>checkpointer</literal>,
      <literal>startup</literal>, <literal>walreceiver</literal>,
      <literal>walsender</literal> and <literal>walwriter</literal>.
     </entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_activity</structname> view will have one row
   per server process, showing information related to
   the current activity of that process.
  </para>

  <note>
   <para>
    The <structfield>wait_event</structfield> and <structfield>state</structfield> columns are
    independent.  If a backend is in the <literal>active</literal> state,
    it may or may not be <literal>waiting</literal> on some event.  If the state
    is <literal>active</literal> and <structfield>wait_event</structfield> is non-null, it
    means that a query is being executed, but is being blocked somewhere
    in the system.
   </para>
  </note>

  <table id="wait-event-table">
   <title><structname>wait_event</structname> Description</title>

    <tgroup cols="3">
      <thead>
       <row>
        <entry>Wait Event Type</entry>
        <entry>Wait Event Name</entry>
        <entry>Description</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry morerows="64"><literal>LWLock</literal></entry>
        <entry><literal>ShmemIndexLock</literal></entry>
        <entry>Waiting to find or allocate space in shared memory.</entry>
       </row>
       <row>
        <entry><literal>OidGenLock</literal></entry>
        <entry>Waiting to allocate or assign an OID.</entry>
       </row>
        <row>
         <entry><literal>XidGenLock</literal></entry>
         <entry>Waiting to allocate or assign a transaction id.</entry>
        </row>
        <row>
         <entry><literal>ProcArrayLock</literal></entry>
         <entry>Waiting to get a snapshot or clearing a transaction id at
         transaction end.</entry>
        </row>
        <row>
         <entry><literal>SInvalReadLock</literal></entry>
         <entry>Waiting to retrieve or remove messages from shared invalidation
         queue.</entry>
        </row>
        <row>
         <entry><literal>SInvalWriteLock</literal></entry>
         <entry>Waiting to add a message in shared invalidation queue.</entry>
        </row>
        <row>
         <entry><literal>WALBufMappingLock</literal></entry>
         <entry>Waiting to replace a page in WAL buffers.</entry>
        </row>
        <row>
         <entry><literal>WALWriteLock</literal></entry>
         <entry>Waiting for WAL buffers to be written to disk.</entry>
        </row>
        <row>
         <entry><literal>ControlFileLock</literal></entry>
         <entry>Waiting to read or update the control file or creation of a
         new WAL file.</entry>
        </row>
        <row>
         <entry><literal>CheckpointLock</literal></entry>
         <entry>Waiting to perform checkpoint.</entry>
        </row>
        <row>
         <entry><literal>CLogControlLock</literal></entry>
         <entry>Waiting to read or update transaction status.</entry>
        </row>
        <row>
         <entry><literal>SubtransControlLock</literal></entry>
         <entry>Waiting to read or update sub-transaction information.</entry>
        </row>
        <row>
         <entry><literal>MultiXactGenLock</literal></entry>
         <entry>Waiting to read or update shared multixact state.</entry>
        </row>
        <row>
         <entry><literal>MultiXactOffsetControlLock</literal></entry>
         <entry>Waiting to read or update multixact offset mappings.</entry>
        </row>
        <row>
         <entry><literal>MultiXactMemberControlLock</literal></entry>
         <entry>Waiting to read or update multixact member mappings.</entry>
        </row>
        <row>
         <entry><literal>RelCacheInitLock</literal></entry>
         <entry>Waiting to read or write relation cache initialization
         file.</entry>
        </row>
        <row>
         <entry><literal>CheckpointerCommLock</literal></entry>
         <entry>Waiting to manage fsync requests.</entry>
        </row>
        <row>
         <entry><literal>TwoPhaseStateLock</literal></entry>
         <entry>Waiting to read or update the state of prepared transactions.</entry>
        </row>
        <row>
         <entry><literal>TablespaceCreateLock</literal></entry>
         <entry>Waiting to create or drop the tablespace.</entry>
        </row>
        <row>
         <entry><literal>BtreeVacuumLock</literal></entry>
          <entry>Waiting to read or update vacuum-related information for a
          B-tree index.</entry>
        </row>
        <row>
         <entry><literal>AddinShmemInitLock</literal></entry>
         <entry>Waiting to manage space allocation in shared memory.</entry>
        </row>
        <row>
         <entry><literal>AutovacuumLock</literal></entry>
         <entry>Autovacuum worker or launcher waiting to update or
         read the current state of autovacuum workers.</entry>
        </row>
        <row>
         <entry><literal>AutovacuumScheduleLock</literal></entry>
         <entry>Waiting to ensure that the table it has selected for a vacuum
         still needs vacuuming.
         </entry>
        </row>
        <row>
         <entry><literal>SyncScanLock</literal></entry>
         <entry>Waiting to get the start location of a scan on a table for
         synchronized scans.</entry>
        </row>
        <row>
         <entry><literal>RelationMappingLock</literal></entry>
         <entry>Waiting to update the relation map file used to store catalog
         to filenode mapping.
         </entry>
        </row>
        <row>
         <entry><literal>AsyncCtlLock</literal></entry>
         <entry>Waiting to read or update shared notification state.</entry>
        </row>
        <row>
         <entry><literal>AsyncQueueLock</literal></entry>
          <entry>Waiting to read or update notification messages.</entry>
        </row>
        <row>
         <entry><literal>SerializableXactHashLock</literal></entry>
         <entry>Waiting to retrieve or store information about serializable
         transactions.</entry>
        </row>
        <row>
         <entry><literal>SerializableFinishedListLock</literal></entry>
         <entry>Waiting to access the list of finished serializable
         transactions.</entry>
        </row>
        <row>
         <entry><literal>SerializablePredicateLockListLock</literal></entry>
         <entry>Waiting to perform an operation on a list of locks held by
         serializable transactions.</entry>
        </row>
        <row>
         <entry><literal>OldSerXidLock</literal></entry>
         <entry>Waiting to read or record conflicting serializable
         transactions.</entry>
        </row>
        <row>
         <entry><literal>SyncRepLock</literal></entry>
         <entry>Waiting to read or update information about synchronous
         replicas.</entry>
        </row>
        <row>
         <entry><literal>BackgroundWorkerLock</literal></entry>
         <entry>Waiting to read or update background worker state.</entry>
        </row>
        <row>
         <entry><literal>DynamicSharedMemoryControlLock</literal></entry>
         <entry>Waiting to read or update dynamic shared memory state.</entry>
        </row>
        <row>
         <entry><literal>AutoFileLock</literal></entry>
         <entry>Waiting to update the <filename>postgresql.auto.conf</filename> file.</entry>
        </row>
        <row>
         <entry><literal>ReplicationSlotAllocationLock</literal></entry>
         <entry>Waiting to allocate or free a replication slot.</entry>
        </row>
        <row>
         <entry><literal>ReplicationSlotControlLock</literal></entry>
         <entry>Waiting to read or update replication slot state.</entry>
        </row>
        <row>
         <entry><literal>CommitTsControlLock</literal></entry>
         <entry>Waiting to read or update transaction commit timestamps.</entry>
        </row>
        <row>
         <entry><literal>CommitTsLock</literal></entry>
         <entry>Waiting to read or update the last value set for the
         transaction timestamp.</entry>
        </row>
        <row>
         <entry><literal>ReplicationOriginLock</literal></entry>
         <entry>Waiting to setup, drop or use replication origin.</entry>
        </row>
        <row>
         <entry><literal>MultiXactTruncationLock</literal></entry>
         <entry>Waiting to read or truncate multixact information.</entry>
        </row>
        <row>
         <entry><literal>OldSnapshotTimeMapLock</literal></entry>
         <entry>Waiting to read or update old snapshot control information.</entry>
        </row>
        <row>
         <entry><literal>BackendRandomLock</literal></entry>
         <entry>Waiting to generate a random number.</entry>
        </row>
        <row>
         <entry><literal>LogicalRepWorkerLock</literal></entry>
         <entry>Waiting for action on logical replication worker to finish.</entry>
        </row>
        <row>
         <entry><literal>CLogTruncationLock</literal></entry>
         <entry>Waiting to truncate the write-ahead log or waiting for write-ahead log truncation to finish.</entry>
        </row>
        <row>
         <entry><literal>clog</literal></entry>
         <entry>Waiting for I/O on a clog (transaction status) buffer.</entry>
        </row>
        <row>
         <entry><literal>commit_timestamp</literal></entry>
         <entry>Waiting for I/O on commit timestamp buffer.</entry>
        </row>
        <row>
         <entry><literal>subtrans</literal></entry>
         <entry>Waiting for I/O a subtransaction buffer.</entry>
        </row>
        <row>
         <entry><literal>multixact_offset</literal></entry>
         <entry>Waiting for I/O on a multixact offset buffer.</entry>
        </row>
        <row>
         <entry><literal>multixact_member</literal></entry>
         <entry>Waiting for I/O on a multixact_member buffer.</entry>
        </row>
        <row>
         <entry><literal>async</literal></entry>
         <entry>Waiting for I/O on an async (notify) buffer.</entry>
        </row>
        <row>
         <entry><literal>oldserxid</literal></entry>
         <entry>Waiting for I/O on an oldserxid buffer.</entry>
        </row>
        <row>
         <entry><literal>wal_insert</literal></entry>
         <entry>Waiting to insert WAL into a memory buffer.</entry>
        </row>
        <row>
         <entry><literal>buffer_content</literal></entry>
         <entry>Waiting to read or write a data page in memory.</entry>
        </row>
        <row>
         <entry><literal>buffer_io</literal></entry>
         <entry>Waiting for I/O on a data page.</entry>
        </row>
        <row>
         <entry><literal>replication_origin</literal></entry>
         <entry>Waiting to read or update the replication progress.</entry>
        </row>
        <row>
         <entry><literal>replication_slot_io</literal></entry>
         <entry>Waiting for I/O on a replication slot.</entry>
        </row>
        <row>
         <entry><literal>proc</literal></entry>
         <entry>Waiting to read or update the fast-path lock information.</entry>
        </row>
        <row>
         <entry><literal>buffer_mapping</literal></entry>
         <entry>Waiting to associate a data block with a buffer in the buffer
         pool.</entry>
        </row>
        <row>
         <entry><literal>lock_manager</literal></entry>
         <entry>Waiting to add or examine locks for backends, or waiting to
         join or exit a locking group (used by parallel query).</entry>
        </row>
        <row>
         <entry><literal>predicate_lock_manager</literal></entry>
         <entry>Waiting to add or examine predicate lock information.</entry>
        </row>
        <row>
         <entry><literal>parallel_query_dsa</literal></entry>
         <entry>Waiting for parallel query dynamic shared memory allocation lock.</entry>
        </row>
        <row>
         <entry><literal>tbm</literal></entry>
         <entry>Waiting for TBM shared iterator lock.</entry>
        </row>
        <row>
         <entry><literal>parallel_append</literal></entry>
         <entry>Waiting to choose the next subplan during Parallel Append plan
         execution.</entry>
        </row>
        <row>
         <entry><literal>parallel_hash_join</literal></entry>
         <entry>Waiting to allocate or exchange a chunk of memory or update
         counters during Parallel Hash plan execution.</entry>
        </row>
        <row>
         <entry morerows="9"><literal>Lock</literal></entry>
         <entry><literal>relation</literal></entry>
         <entry>Waiting to acquire a lock on a relation.</entry>
        </row>
        <row>
         <entry><literal>extend</literal></entry>
         <entry>Waiting to extend a relation.</entry>
        </row>
        <row>
         <entry><literal>page</literal></entry>
         <entry>Waiting to acquire a lock on page of a relation.</entry>
        </row>
        <row>
         <entry><literal>tuple</literal></entry>
         <entry>Waiting to acquire a lock on a tuple.</entry>
        </row>
        <row>
         <entry><literal>transactionid</literal></entry>
         <entry>Waiting for a transaction to finish.</entry>
        </row>
        <row>
         <entry><literal>virtualxid</literal></entry>
         <entry>Waiting to acquire a virtual xid lock.</entry>
        </row>
        <row>
         <entry><literal>speculative token</literal></entry>
         <entry>Waiting to acquire a speculative insertion lock.</entry>
        </row>
        <row>
         <entry><literal>object</literal></entry>
         <entry>Waiting to acquire a lock on a non-relation database object.</entry>
        </row>
        <row>
         <entry><literal>userlock</literal></entry>
         <entry>Waiting to acquire a user lock.</entry>
        </row>
        <row>
         <entry><literal>advisory</literal></entry>
         <entry>Waiting to acquire an advisory user lock.</entry>
        </row>
        <row>
         <entry><literal>BufferPin</literal></entry>
         <entry><literal>BufferPin</literal></entry>
         <entry>Waiting to acquire a pin on a buffer.</entry>
        </row>
        <row>
         <entry morerows="13"><literal>Activity</literal></entry>
         <entry><literal>ArchiverMain</literal></entry>
         <entry>Waiting in main loop of the archiver process.</entry>
        </row>
        <row>
         <entry><literal>AutoVacuumMain</literal></entry>
         <entry>Waiting in main loop of autovacuum launcher process.</entry>
        </row>
        <row>
         <entry><literal>BgWriterHibernate</literal></entry>
         <entry>Waiting in background writer process, hibernating.</entry>
        </row>
        <row>
         <entry><literal>BgWriterMain</literal></entry>
         <entry>Waiting in main loop of background writer process background worker.</entry>
        </row>
        <row>
         <entry><literal>CheckpointerMain</literal></entry>
         <entry>Waiting in main loop of checkpointer process.</entry>
        </row>
        <row>
         <entry><literal>LogicalLauncherMain</literal></entry>
         <entry>Waiting in main loop of logical launcher process.</entry>
        </row>
        <row>
         <entry><literal>LogicalApplyMain</literal></entry>
         <entry>Waiting in main loop of logical apply process.</entry>
        </row>
        <row>
         <entry><literal>PgStatMain</literal></entry>
         <entry>Waiting in main loop of the statistics collector process.</entry>
        </row>
        <row>
         <entry><literal>RecoveryWalAll</literal></entry>
         <entry>Waiting for WAL from any kind of source (local, archive or stream) at recovery.</entry>
        </row>
        <row>
         <entry><literal>RecoveryWalStream</literal></entry>
         <entry>Waiting for WAL from a stream at recovery.</entry>
        </row>
        <row>
         <entry><literal>SysLoggerMain</literal></entry>
         <entry>Waiting in main loop of syslogger process.</entry>
        </row>
        <row>
         <entry><literal>WalReceiverMain</literal></entry>
         <entry>Waiting in main loop of WAL receiver process.</entry>
        </row>
        <row>
         <entry><literal>WalSenderMain</literal></entry>
         <entry>Waiting in main loop of WAL sender process.</entry>
        </row>
        <row>
         <entry><literal>WalWriterMain</literal></entry>
         <entry>Waiting in main loop of WAL writer process.</entry>
        </row>
        <row>
         <entry morerows="7"><literal>Client</literal></entry>
         <entry><literal>ClientRead</literal></entry>
         <entry>Waiting to read data from the client.</entry>
        </row>
        <row>
         <entry><literal>ClientWrite</literal></entry>
         <entry>Waiting to write data from the client.</entry>
        </row>
        <row>
         <entry><literal>LibPQWalReceiverConnect</literal></entry>
         <entry>Waiting in WAL receiver to establish connection to remote server.</entry>
        </row>
        <row>
         <entry><literal>LibPQWalReceiverReceive</literal></entry>
         <entry>Waiting in WAL receiver to receive data from remote server.</entry>
        </row>
        <row>
         <entry><literal>SSLOpenServer</literal></entry>
         <entry>Waiting for SSL while attempting connection.</entry>
        </row>
        <row>
         <entry><literal>WalReceiverWaitStart</literal></entry>
         <entry>Waiting for startup process to send initial data for streaming replication.</entry>
        </row>
        <row>
         <entry><literal>WalSenderWaitForWAL</literal></entry>
         <entry>Waiting for WAL to be flushed in WAL sender process.</entry>
        </row>
        <row>
         <entry><literal>WalSenderWriteData</literal></entry>
         <entry>Waiting for any activity when processing replies from WAL receiver in WAL sender process.</entry>
        </row>
        <row>
         <entry><literal>Extension</literal></entry>
         <entry><literal>Extension</literal></entry>
         <entry>Waiting in an extension.</entry>
        </row>
        <row>
         <entry morerows="33"><literal>IPC</literal></entry>
         <entry><literal>BgWorkerShutdown</literal></entry>
         <entry>Waiting for background worker to shut down.</entry>
        </row>
        <row>
         <entry><literal>BgWorkerStartup</literal></entry>
         <entry>Waiting for background worker to start up.</entry>
        </row>
        <row>
         <entry><literal>BtreePage</literal></entry>
         <entry>Waiting for the page number needed to continue a parallel B-tree scan to become available.</entry>
        </row>
        <row>
         <entry><literal>ExecuteGather</literal></entry>
         <entry>Waiting for activity from child process when executing <literal>Gather</literal> node.</entry>
        </row>
        <row>
          <entry><literal>Hash/Batch/Allocating</literal></entry>
          <entry>Waiting for an elected Parallel Hash participant to allocate a hash table.</entry>
        </row>
        <row>
          <entry><literal>Hash/Batch/Electing</literal></entry>
          <entry>Electing a Parallel Hash participant to allocate a hash table.</entry>
        </row>
        <row>
          <entry><literal>Hash/Batch/Loading</literal></entry>
          <entry>Waiting for other Parallel Hash participants to finish loading a hash table.</entry>
        </row>
        <row>
          <entry><literal>Hash/Build/Allocating</literal></entry>
          <entry>Waiting for an elected Parallel Hash participant to allocate the initial hash table.</entry>
        </row>
        <row>
          <entry><literal>Hash/Build/Electing</literal></entry>
          <entry>Electing a Parallel Hash participant to allocate the initial hash table.</entry>
        </row>
        <row>
          <entry><literal>Hash/Build/HashingInner</literal></entry>
          <entry>Waiting for other Parallel Hash participants to finish hashing the inner relation.</entry>
        </row>
        <row>
          <entry><literal>Hash/Build/HashingOuter</literal></entry>
          <entry>Waiting for other Parallel Hash participants to finish partitioning the outer relation.</entry>
        </row>
        <row>
          <entry><literal>Hash/GrowBatches/Allocating</literal></entry>
          <entry>Waiting for an elected Parallel Hash participant to allocate more batches.</entry>
        </row>
        <row>
          <entry><literal>Hash/GrowBatches/Deciding</literal></entry>
          <entry>Electing a Parallel Hash participant to decide on future batch growth.</entry>
        </row>
        <row>
          <entry><literal>Hash/GrowBatches/Electing</literal></entry>
          <entry>Electing a Parallel Hash participant to allocate more batches.</entry>
        </row>
        <row>
          <entry><literal>Hash/GrowBatches/Finishing</literal></entry>
          <entry>Waiting for an elected Parallel Hash participant to decide on future batch growth.</entry>
        </row>
        <row>
          <entry><literal>Hash/GrowBatches/Repartitioning</literal></entry>
          <entry>Waiting for other Parallel Hash participants to finishing repartitioning.</entry>
        </row>
        <row>
          <entry><literal>Hash/GrowBuckets/Allocating</literal></entry>
          <entry>Waiting for an elected Parallel Hash participant to finish allocating more buckets.</entry>
        </row>
        <row>
          <entry><literal>Hash/GrowBuckets/Electing</literal></entry>
          <entry>Electing a Parallel Hash participant to allocate more buckets.</entry>
        </row>
        <row>
          <entry><literal>Hash/GrowBuckets/Reinserting</literal></entry>
          <entry>Waiting for other Parallel Hash participants to finish inserting tuples into new buckets.</entry>
        </row>
        <row>
         <entry><literal>LogicalSyncData</literal></entry>
         <entry>Waiting for logical replication remote server to send data for initial table synchronization.</entry>
        </row>
        <row>
         <entry><literal>LogicalSyncStateChange</literal></entry>
         <entry>Waiting for logical replication remote server to change state.</entry>
        </row>
        <row>
         <entry><literal>MessageQueueInternal</literal></entry>
         <entry>Waiting for other process to be attached in shared message queue.</entry>
        </row>
        <row>
         <entry><literal>MessageQueuePutMessage</literal></entry>
         <entry>Waiting to write a protocol message to a shared message queue.</entry>
        </row>
        <row>
         <entry><literal>MessageQueueReceive</literal></entry>
         <entry>Waiting to receive bytes from a shared message queue.</entry>
        </row>
        <row>
         <entry><literal>MessageQueueSend</literal></entry>
         <entry>Waiting to send bytes to a shared message queue.</entry>
        </row>
        <row>
         <entry><literal>ParallelFinish</literal></entry>
         <entry>Waiting for parallel workers to finish computing.</entry>
        </row>
        <row>
         <entry><literal>ParallelBitmapScan</literal></entry>
         <entry>Waiting for parallel bitmap scan to become initialized.</entry>
        </row>
        <row>
         <entry><literal>ParallelCreateIndexScan</literal></entry>
         <entry>Waiting for parallel <command>CREATE INDEX</command> workers to finish heap scan.</entry>
        </row>
        <row>
         <entry><literal>ProcArrayGroupUpdate</literal></entry>
         <entry>Waiting for group leader to clear transaction id at transaction end.</entry>
        </row>
        <row>
         <entry><literal>ClogGroupUpdate</literal></entry>
         <entry>Waiting for group leader to update transaction status at transaction end.</entry>
        </row>
        <row>
         <entry><literal>ReplicationOriginDrop</literal></entry>
         <entry>Waiting for a replication origin to become inactive to be dropped.</entry>
        </row>
        <row>
         <entry><literal>ReplicationSlotDrop</literal></entry>
         <entry>Waiting for a replication slot to become inactive to be dropped.</entry>
        </row>
        <row>
         <entry><literal>SafeSnapshot</literal></entry>
         <entry>Waiting for a snapshot for a <literal>READ ONLY DEFERRABLE</literal> transaction.</entry>
        </row>
        <row>
         <entry><literal>SyncRep</literal></entry>
         <entry>Waiting for confirmation from remote server during synchronous replication.</entry>
        </row>
        <row>
         <entry morerows="2"><literal>Timeout</literal></entry>
         <entry><literal>BaseBackupThrottle</literal></entry>
         <entry>Waiting during base backup when throttling activity.</entry>
        </row>
        <row>
         <entry><literal>PgSleep</literal></entry>
         <entry>Waiting in process that called <function>pg_sleep</function>.</entry>
        </row>
        <row>
         <entry><literal>RecoveryApplyDelay</literal></entry>
         <entry>Waiting to apply WAL at recovery because it is delayed.</entry>
        </row>
        <row>
         <entry morerows="65"><literal>IO</literal></entry>
         <entry><literal>BufFileRead</literal></entry>
         <entry>Waiting for a read from a buffered file.</entry>
        </row>
        <row>
         <entry><literal>BufFileWrite</literal></entry>
         <entry>Waiting for a write to a buffered file.</entry>
        </row>
        <row>
         <entry><literal>ControlFileRead</literal></entry>
         <entry>Waiting for a read from the control file.</entry>
        </row>
        <row>
         <entry><literal>ControlFileSync</literal></entry>
         <entry>Waiting for the control file to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>ControlFileSyncUpdate</literal></entry>
         <entry>Waiting for an update to the control file to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>ControlFileWrite</literal></entry>
         <entry>Waiting for a write to the control file.</entry>
        </row>
        <row>
         <entry><literal>ControlFileWriteUpdate</literal></entry>
         <entry>Waiting for a write to update the control file.</entry>
        </row>
        <row>
         <entry><literal>CopyFileRead</literal></entry>
         <entry>Waiting for a read during a file copy operation.</entry>
        </row>
        <row>
         <entry><literal>CopyFileWrite</literal></entry>
         <entry>Waiting for a write during a file copy operation.</entry>
        </row>
        <row>
         <entry><literal>DataFileExtend</literal></entry>
         <entry>Waiting for a relation data file to be extended.</entry>
        </row>
        <row>
         <entry><literal>DataFileFlush</literal></entry>
         <entry>Waiting for a relation data file to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>DataFileImmediateSync</literal></entry>
         <entry>Waiting for an immediate synchronization of a relation data file to stable storage.</entry>
        </row>
        <row>
         <entry><literal>DataFilePrefetch</literal></entry>
         <entry>Waiting for an asynchronous prefetch from a relation data file.</entry>
        </row>
        <row>
         <entry><literal>DataFileRead</literal></entry>
         <entry>Waiting for a read from a relation data file.</entry>
        </row>
        <row>
         <entry><literal>DataFileSync</literal></entry>
         <entry>Waiting for changes to a relation data file to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>DataFileTruncate</literal></entry>
         <entry>Waiting for a relation data file to be truncated.</entry>
        </row>
        <row>
         <entry><literal>DataFileWrite</literal></entry>
         <entry>Waiting for a write to a relation data file.</entry>
        </row>
        <row>
         <entry><literal>DSMFillZeroWrite</literal></entry>
         <entry>Waiting to write zero bytes to a dynamic shared memory backing file.</entry>
        </row>
        <row>
         <entry><literal>LockFileAddToDataDirRead</literal></entry>
         <entry>Waiting for a read while adding a line to the data directory lock file.</entry>
        </row>
        <row>
         <entry><literal>LockFileAddToDataDirSync</literal></entry>
         <entry>Waiting for data to reach stable storage while adding a line to the data directory lock file.</entry>
        </row>
        <row>
         <entry><literal>LockFileAddToDataDirWrite</literal></entry>
         <entry>Waiting for a write while adding a line to the data directory lock file.</entry>
        </row>
        <row>
         <entry><literal>LockFileCreateRead</literal></entry>
         <entry>Waiting to read while creating the data directory lock file.</entry>
        </row>
        <row>
         <entry><literal>LockFileCreateSync</literal></entry>
         <entry>Waiting for data to reach stable storage while creating the data directory lock file.</entry>
        </row>
        <row>
         <entry><literal>LockFileCreateWrite</literal></entry>
         <entry>Waiting for a write while creating the data directory lock file.</entry>
        </row>
        <row>
         <entry><literal>LockFileReCheckDataDirRead</literal></entry>
         <entry>Waiting for a read during recheck of the data directory lock file.</entry>
        </row>
        <row>
         <entry><literal>LogicalRewriteCheckpointSync</literal></entry>
         <entry>Waiting for logical rewrite mappings to reach stable storage during a checkpoint.</entry>
        </row>
        <row>
         <entry><literal>LogicalRewriteMappingSync</literal></entry>
         <entry>Waiting for mapping data to reach stable storage during a logical rewrite.</entry>
        </row>
        <row>
         <entry><literal>LogicalRewriteMappingWrite</literal></entry>
         <entry>Waiting for a write of mapping data during a logical rewrite.</entry>
        </row>
        <row>
         <entry><literal>LogicalRewriteSync</literal></entry>
         <entry>Waiting for logical rewrite mappings to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>LogicalRewriteWrite</literal></entry>
         <entry>Waiting for a write of logical rewrite mappings.</entry>
        </row>
        <row>
         <entry><literal>RelationMapRead</literal></entry>
         <entry>Waiting for a read of the relation map file.</entry>
        </row>
        <row>
         <entry><literal>RelationMapSync</literal></entry>
         <entry>Waiting for the relation map file to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>RelationMapWrite</literal></entry>
         <entry>Waiting for a write to the relation map file.</entry>
        </row>
        <row>
         <entry><literal>ReorderBufferRead</literal></entry>
         <entry>Waiting for a read during reorder buffer management.</entry>
        </row>
        <row>
         <entry><literal>ReorderBufferWrite</literal></entry>
         <entry>Waiting for a write during reorder buffer management.</entry>
        </row>
        <row>
         <entry><literal>ReorderLogicalMappingRead</literal></entry>
         <entry>Waiting for a read of a logical mapping during reorder buffer management.</entry>
        </row>
        <row>
         <entry><literal>ReplicationSlotRead</literal></entry>
         <entry>Waiting for a read from a replication slot control file.</entry>
        </row>
        <row>
         <entry><literal>ReplicationSlotRestoreSync</literal></entry>
         <entry>Waiting for a replication slot control file to reach stable storage while restoring it to memory.</entry>
        </row>
        <row>
         <entry><literal>ReplicationSlotSync</literal></entry>
         <entry>Waiting for a replication slot control file to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>ReplicationSlotWrite</literal></entry>
         <entry>Waiting for a write to a replication slot control file.</entry>
        </row>
        <row>
         <entry><literal>SLRUFlushSync</literal></entry>
         <entry>Waiting for SLRU data to reach stable storage during a checkpoint or database shutdown.</entry>
        </row>
        <row>
         <entry><literal>SLRURead</literal></entry>
         <entry>Waiting for a read of an SLRU page.</entry>
        </row>
        <row>
         <entry><literal>SLRUSync</literal></entry>
         <entry>Waiting for SLRU data to reach stable storage following a page write.</entry>
        </row>
        <row>
         <entry><literal>SLRUWrite</literal></entry>
         <entry>Waiting for a write of an SLRU page.</entry>
        </row>
        <row>
         <entry><literal>SnapbuildRead</literal></entry>
         <entry>Waiting for a read of a serialized historical catalog snapshot.</entry>
        </row>
        <row>
         <entry><literal>SnapbuildSync</literal></entry>
         <entry>Waiting for a serialized historical catalog snapshot to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>SnapbuildWrite</literal></entry>
         <entry>Waiting for a write of a serialized historical catalog snapshot.</entry>
        </row>
        <row>
         <entry><literal>TimelineHistoryFileSync</literal></entry>
         <entry>Waiting for a timeline history file received via streaming replication to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>TimelineHistoryFileWrite</literal></entry>
         <entry>Waiting for a write of a timeline history file received via streaming replication.</entry>
        </row>
        <row>
         <entry><literal>TimelineHistoryRead</literal></entry>
         <entry>Waiting for a read of a timeline history file.</entry>
        </row>
        <row>
         <entry><literal>TimelineHistorySync</literal></entry>
         <entry>Waiting for a newly created timeline history file to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>TimelineHistoryWrite</literal></entry>
         <entry>Waiting for a write of a newly created timeline history file.</entry>
        </row>
        <row>
         <entry><literal>TwophaseFileRead</literal></entry>
         <entry>Waiting for a read of a two phase state file.</entry>
        </row>
        <row>
         <entry><literal>TwophaseFileSync</literal></entry>
         <entry>Waiting for a two phase state file to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>TwophaseFileWrite</literal></entry>
         <entry>Waiting for a write of a two phase state file.</entry>
        </row>
        <row>
         <entry><literal>WALBootstrapSync</literal></entry>
         <entry>Waiting for WAL to reach stable storage during bootstrapping.</entry>
        </row>
        <row>
         <entry><literal>WALBootstrapWrite</literal></entry>
         <entry>Waiting for a write of a WAL page during bootstrapping.</entry>
        </row>
        <row>
         <entry><literal>WALCopyRead</literal></entry>
         <entry>Waiting for a read when creating a new WAL segment by copying an existing one.</entry>
        </row>
        <row>
         <entry><literal>WALCopySync</literal></entry>
         <entry>Waiting a new WAL segment created by copying an existing one to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>WALCopyWrite</literal></entry>
         <entry>Waiting for a write when creating a new WAL segment by copying an existing one.</entry>
        </row>
        <row>
         <entry><literal>WALInitSync</literal></entry>
         <entry>Waiting for a newly initialized WAL file to reach stable storage.</entry>
        </row>
        <row>
         <entry><literal>WALInitWrite</literal></entry>
         <entry>Waiting for a write while initializing a new WAL file.</entry>
        </row>
        <row>
         <entry><literal>WALRead</literal></entry>
         <entry>Waiting for a read from a WAL file.</entry>
        </row>
        <row>
         <entry><literal>WALSenderTimelineHistoryRead</literal></entry>
         <entry>Waiting for a read from a timeline history file during walsender timeline command.</entry>
        </row>
        <row>
         <entry><literal>WALSyncMethodAssign</literal></entry>
         <entry>Waiting for data to reach stable storage while assigning WAL sync method.</entry>
        </row>
        <row>
         <entry><literal>WALWrite</literal></entry>
         <entry>Waiting for a write to a WAL file.</entry>
        </row>
      </tbody>
     </tgroup>
    </table>

   <note>
    <para>
     For tranches registered by extensions, the name is specified by extension
     and this will be displayed as <structfield>wait_event</structfield>.  It is quite
     possible that user has registered the tranche in one of the backends (by
     having allocation in dynamic shared memory) in which case other backends
     won't have that information, so we display <literal>extension</literal> for such
     cases.
    </para>
   </note>

   <para>
     Here is an example of how wait events can be viewed

<programlisting>
SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL;
 pid  | wait_event_type |  wait_event
------+-----------------+---------------
 2540 | Lock            | relation
 6644 | LWLock          | ProcArrayLock
(2 rows)
</programlisting>
   </para>

  <table id="pg-stat-replication-view" xreflabel="pg_stat_replication">
   <title><structname>pg_stat_replication</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>pid</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>Process ID of a WAL sender process</entry>
    </row>
    <row>
     <entry><structfield>usesysid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of the user logged into this WAL sender process</entry>
    </row>
    <row>
     <entry><structfield>usename</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the user logged into this WAL sender process</entry>
    </row>
    <row>
     <entry><structfield>application_name</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Name of the application that is connected
      to this WAL sender</entry>
    </row>
    <row>
     <entry><structfield>client_addr</structfield></entry>
     <entry><type>inet</type></entry>
     <entry>IP address of the client connected to this WAL sender.
      If this field is null, it indicates that the client is
      connected via a Unix socket on the server machine.
     </entry>
    </row>
    <row>
     <entry><structfield>client_hostname</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Host name of the connected client, as reported by a
      reverse DNS lookup of <structfield>client_addr</structfield>. This field will
      only be non-null for IP connections, and only when <xref
      linkend="guc-log-hostname"/> is enabled.
     </entry>
    </row>
    <row>
     <entry><structfield>client_port</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>TCP port number that the client is using for communication
      with this WAL sender, or <literal>-1</literal> if a Unix socket is used
     </entry>
    </row>
    <row>
     <entry><structfield>backend_start</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Time when this process was started, i.e., when the
      client connected to this WAL sender
     </entry>
    </row>
    <row>
     <entry><structfield>backend_xmin</structfield></entry>
     <entry><type>xid</type></entry>
     <entry>This standby's <literal>xmin</literal> horizon reported
     by <xref linkend="guc-hot-standby-feedback"/>.</entry>
    </row>
    <row>
     <entry><structfield>state</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Current WAL sender state.
       Possible values are:
       <itemizedlist>
         <listitem>
          <para>
           <literal>startup</literal>: This WAL sender is starting up.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>catchup</literal>: This WAL sender's connected standby is
           catching up with the primary.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>streaming</literal>: This WAL sender is streaming changes
           after its connected standby server has caught up with the primary.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>backup</literal>: This WAL sender is sending a backup.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>stopping</literal>: This WAL sender is stopping.
          </para>
         </listitem>
       </itemizedlist>
     </entry>
    </row>
    <row>
     <entry><structfield>sent_lsn</structfield></entry>
     <entry><type>pg_lsn</type></entry>
     <entry>Last write-ahead log location sent on this connection</entry>
    </row>
    <row>
     <entry><structfield>write_lsn</structfield></entry>
     <entry><type>pg_lsn</type></entry>
     <entry>Last write-ahead log location written to disk by this standby
      server</entry>
    </row>
    <row>
     <entry><structfield>flush_lsn</structfield></entry>
     <entry><type>pg_lsn</type></entry>
     <entry>Last write-ahead log location flushed to disk by this standby
      server</entry>
    </row>
    <row>
     <entry><structfield>replay_lsn</structfield></entry>
     <entry><type>pg_lsn</type></entry>
     <entry>Last write-ahead log location replayed into the database on this
      standby server</entry>
    </row>
    <row>
     <entry><structfield>write_lag</structfield></entry>
     <entry><type>interval</type></entry>
     <entry>Time elapsed between flushing recent WAL locally and receiving
      notification that this standby server has written it (but not yet
      flushed it or applied it).  This can be used to gauge the delay that
      <literal>synchronous_commit</literal> level
      <literal>remote_write</literal> incurred while committing if this
      server was configured as a synchronous standby.</entry>
    </row>
    <row>
     <entry><structfield>flush_lag</structfield></entry>
     <entry><type>interval</type></entry>
     <entry>Time elapsed between flushing recent WAL locally and receiving
      notification that this standby server has written and flushed it
      (but not yet applied it).  This can be used to gauge the delay that
      <literal>synchronous_commit</literal> level
      <literal>remote_flush</literal> incurred while committing if this
      server was configured as a synchronous standby.</entry>
    </row>
    <row>
     <entry><structfield>replay_lag</structfield></entry>
     <entry><type>interval</type></entry>
     <entry>Time elapsed between flushing recent WAL locally and receiving
      notification that this standby server has written, flushed and
      applied it.  This can be used to gauge the delay that
      <literal>synchronous_commit</literal> level
      <literal>remote_apply</literal> incurred while committing if this
      server was configured as a synchronous standby.</entry>
    </row>
    <row>
     <entry><structfield>sync_priority</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>Priority of this standby server for being chosen as the
      synchronous standby in a priority-based synchronous replication.
      This has no effect in a quorum-based synchronous replication.</entry>
    </row>
    <row>
     <entry><structfield>sync_state</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Synchronous state of this standby server.
       Possible values are:
       <itemizedlist>
         <listitem>
          <para>
           <literal>async</literal>: This standby server is asynchronous.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>potential</literal>: This standby server is now asynchronous,
           but can potentially become synchronous if one of current
           synchronous ones fails.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>sync</literal>: This standby server is synchronous.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>quorum</literal>: This standby server is considered as a candidate
           for quorum standbys.
          </para>
         </listitem>
       </itemizedlist>
     </entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_replication</structname> view will contain one row
   per WAL sender process, showing statistics about replication to that
   sender's connected standby server.  Only directly connected standbys are
   listed; no information is available about downstream standby servers.
  </para>

  <para>
   The lag times reported in the <structname>pg_stat_replication</structname>
   view are measurements of the time taken for recent WAL to be written,
   flushed and replayed and for the sender to know about it.  These times
   represent the commit delay that was (or would have been) introduced by each
   synchronous commit level, if the remote server was configured as a
   synchronous standby.  For an asynchronous standby, the
   <structfield>replay_lag</structfield> column approximates the delay
   before recent transactions became visible to queries.  If the standby
   server has entirely caught up with the sending server and there is no more
   WAL activity, the most recently measured lag times will continue to be
   displayed for a short time and then show NULL.
  </para>

  <para>
   Lag times work automatically for physical replication. Logical decoding
   plugins may optionally emit tracking messages; if they do not, the tracking
   mechanism will simply display NULL lag.
  </para>

  <note>
   <para>
    The reported lag times are not predictions of how long it will take for
    the standby to catch up with the sending server assuming the current
    rate of replay.  Such a system would show similar times while new WAL is
    being generated, but would differ when the sender becomes idle.  In
    particular, when the standby has caught up completely,
    <structname>pg_stat_replication</structname> shows the time taken to
    write, flush and replay the most recent reported WAL location rather than
    zero as some users might expect.  This is consistent with the goal of
    measuring synchronous commit and transaction visibility delays for
    recent write transactions.
    To reduce confusion for users expecting a different model of lag, the
    lag columns revert to NULL after a short time on a fully replayed idle
    system. Monitoring systems should choose whether to represent this
    as missing data, zero or continue to display the last known value.
   </para>
  </note>

  <table id="pg-stat-wal-receiver-view" xreflabel="pg_stat_wal_receiver">
   <title><structname>pg_stat_wal_receiver</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>pid</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>Process ID of the WAL receiver process</entry>
    </row>
    <row>
     <entry><structfield>status</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Activity status of the WAL receiver process</entry>
    </row>
    <row>
     <entry><structfield>receive_start_lsn</structfield></entry>
     <entry><type>pg_lsn</type></entry>
     <entry>First write-ahead log location used when WAL receiver is
      started</entry>
    </row>
    <row>
     <entry><structfield>receive_start_tli</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>First timeline number used when WAL receiver is started</entry>
    </row>
    <row>
     <entry><structfield>received_lsn</structfield></entry>
     <entry><type>pg_lsn</type></entry>
     <entry>Last write-ahead log location already received and flushed to
      disk, the initial value of this field being the first log location used
      when WAL receiver is started</entry>
    </row>
    <row>
     <entry><structfield>received_tli</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>Timeline number of last write-ahead log location received and
      flushed to disk, the initial value of this field being the timeline
      number of the first log location used when WAL receiver is started
     </entry>
    </row>
    <row>
     <entry><structfield>last_msg_send_time</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Send time of last message received from origin WAL sender</entry>
    </row>
    <row>
     <entry><structfield>last_msg_receipt_time</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Receipt time of last message received from origin WAL sender</entry>
    </row>
    <row>
     <entry><structfield>latest_end_lsn</structfield></entry>
     <entry><type>pg_lsn</type></entry>
     <entry>Last write-ahead log location reported to origin WAL sender</entry>
    </row>
    <row>
     <entry><structfield>latest_end_time</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Time of last write-ahead log location reported to origin WAL sender</entry>
    </row>
    <row>
     <entry><structfield>slot_name</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Replication slot name used by this WAL receiver</entry>
    </row>
    <row>
     <entry><structfield>conninfo</structfield></entry>
     <entry><type>text</type></entry>
     <entry>
      Connection string used by this WAL receiver,
      with security-sensitive fields obfuscated.
     </entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_wal_receiver</structname> view will contain only
   one row, showing statistics about the WAL receiver from that receiver's
   connected server.
  </para>

  <table id="pg-stat-subscription" xreflabel="pg_stat_subscription">
   <title><structname>pg_stat_subscription</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>subid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of the subscription</entry>
    </row>
    <row>
     <entry><structfield>subname</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Name of the subscription</entry>
    </row>
    <row>
     <entry><structfield>pid</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>Process ID of the subscription worker process</entry>
    </row>
    <row>
     <entry><structfield>relid</structfield></entry>
     <entry><type>Oid</type></entry>
     <entry>OID of the relation that the worker is synchronizing; null for the
     main apply worker</entry>
    </row>
    <row>
     <entry><structfield>received_lsn</structfield></entry>
     <entry><type>pg_lsn</type></entry>
     <entry>Last write-ahead log location received, the initial value of
      this field being 0</entry>
    </row>
    <row>
     <entry><structfield>last_msg_send_time</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Send time of last message received from origin WAL sender</entry>
    </row>
    <row>
     <entry><structfield>last_msg_receipt_time</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Receipt time of last message received from origin WAL sender
     </entry>
    </row>
    <row>
     <entry><structfield>latest_end_lsn</structfield></entry>
     <entry><type>pg_lsn</type></entry>
     <entry>Last write-ahead log location reported to origin WAL sender
     </entry>
    </row>
    <row>
     <entry><structfield>latest_end_time</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Time of last write-ahead log location reported to origin WAL
      sender</entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_subscription</structname> view will contain one
   row per subscription for main worker (with null PID if the worker is
   not running), and additional rows for workers handling the initial data
   copy of the subscribed tables.
  </para>

  <table id="pg-stat-ssl-view" xreflabel="pg_stat_ssl">
   <title><structname>pg_stat_ssl</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>pid</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>Process ID of a backend or WAL sender process</entry>
    </row>
    <row>
     <entry><structfield>ssl</structfield></entry>
     <entry><type>boolean</type></entry>
     <entry>True if SSL is used on this connection</entry>
    </row>
    <row>
     <entry><structfield>version</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Version of SSL in use, or NULL if SSL is not in use
      on this connection</entry>
    </row>
    <row>
     <entry><structfield>cipher</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Name of SSL cipher in use, or NULL if SSL is not in use
      on this connection</entry>
    </row>
    <row>
     <entry><structfield>bits</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>Number of bits in the encryption algorithm used, or NULL
     if SSL is not used on this connection</entry>
    </row>
    <row>
     <entry><structfield>compression</structfield></entry>
     <entry><type>boolean</type></entry>
     <entry>True if SSL compression is in use, false if not,
      or NULL if SSL is not in use on this connection</entry>
    </row>
    <row>
     <entry><structfield>clientdn</structfield></entry>
     <entry><type>text</type></entry>
     <entry>Distinguished Name (DN) field from the client certificate
      used, or NULL if no client certificate was supplied or if SSL
      is not in use on this connection. This field is truncated if the
      DN field is longer than <symbol>NAMEDATALEN</symbol> (64 characters
      in a standard build)
     </entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_ssl</structname> view will contain one row per
   backend or WAL sender process, showing statistics about SSL usage on
   this connection. It can be joined to <structname>pg_stat_activity</structname>
   or <structname>pg_stat_replication</structname> on the
   <structfield>pid</structfield> column to get more details about the
   connection.
  </para>


  <table id="pg-stat-archiver-view" xreflabel="pg_stat_archiver">
   <title><structname>pg_stat_archiver</structname> View</title>

   <tgroup cols="3">
    <thead>
     <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry><structfield>archived_count</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry>Number of WAL files that have been successfully archived</entry>
     </row>
     <row>
      <entry><structfield>last_archived_wal</structfield></entry>
      <entry><type>text</type></entry>
      <entry>Name of the last WAL file successfully archived</entry>
     </row>
     <row>
      <entry><structfield>last_archived_time</structfield></entry>
      <entry><type>timestamp with time zone</type></entry>
      <entry>Time of the last successful archive operation</entry>
     </row>
     <row>
      <entry><structfield>failed_count</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry>Number of failed attempts for archiving WAL files</entry>
     </row>
     <row>
      <entry><structfield>last_failed_wal</structfield></entry>
      <entry><type>text</type></entry>
      <entry>Name of the WAL file of the last failed archival operation</entry>
     </row>
     <row>
      <entry><structfield>last_failed_time</structfield></entry>
      <entry><type>timestamp with time zone</type></entry>
      <entry>Time of the last failed archival operation</entry>
     </row>
     <row>
      <entry><structfield>stats_reset</structfield></entry>
      <entry><type>timestamp with time zone</type></entry>
      <entry>Time at which these statistics were last reset</entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_archiver</structname> view will always have a
   single row, containing data about the archiver process of the cluster.
  </para>

  <table id="pg-stat-bgwriter-view" xreflabel="pg_stat_bgwriter">
   <title><structname>pg_stat_bgwriter</structname> View</title>

   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry><structfield>checkpoints_timed</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry>Number of scheduled checkpoints that have been performed</entry>
     </row>
     <row>
      <entry><structfield>checkpoints_req</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry>Number of requested checkpoints that have been performed</entry>
     </row>
     <row>
      <entry><structfield>checkpoint_write_time</structfield></entry>
      <entry><type>double precision</type></entry>
      <entry>
        Total amount of time that has been spent in the portion of
        checkpoint processing where files are written to disk, in milliseconds
      </entry>
     </row>
     <row>
      <entry><structfield>checkpoint_sync_time</structfield></entry>
      <entry><type>double precision</type></entry>
      <entry>
        Total amount of time that has been spent in the portion of
        checkpoint processing where files are synchronized to disk, in
        milliseconds
      </entry>
     </row>
     <row>
      <entry><structfield>buffers_checkpoint</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry>Number of buffers written during checkpoints</entry>
     </row>
     <row>
      <entry><structfield>buffers_clean</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry>Number of buffers written by the background writer</entry>
     </row>
     <row>
      <entry><structfield>maxwritten_clean</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry>Number of times the background writer stopped a cleaning
       scan because it had written too many buffers</entry>
     </row>
     <row>
      <entry><structfield>buffers_backend</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry>Number of buffers written directly by a backend</entry>
     </row>
     <row>
      <entry><structfield>buffers_backend_fsync</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry>Number of times a backend had to execute its own
       <function>fsync</function> call (normally the background writer handles those
       even when the backend does its own write)</entry>
     </row>
     <row>
      <entry><structfield>buffers_alloc</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry>Number of buffers allocated</entry>
     </row>
     <row>
      <entry><structfield>stats_reset</structfield></entry>
      <entry><type>timestamp with time zone</type></entry>
      <entry>Time at which these statistics were last reset</entry>
     </row>
    </tbody>
    </tgroup>
  </table>

  <para>
   The <structname>pg_stat_bgwriter</structname> view will always have a
   single row, containing global data for the cluster.
  </para>

  <table id="pg-stat-database-view" xreflabel="pg_stat_database">
   <title><structname>pg_stat_database</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>datid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of a database</entry>
    </row>
    <row>
     <entry><structfield>datname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of this database</entry>
    </row>
    <row>
     <entry><structfield>numbackends</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>Number of backends currently connected to this database.
     This is the only column in this view that returns a value reflecting
     current state; all other columns return the accumulated values since
     the last reset.</entry>
    </row>
    <row>
     <entry><structfield>xact_commit</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of transactions in this database that have been
      committed</entry>
    </row>
    <row>
     <entry><structfield>xact_rollback</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of transactions in this database that have been
      rolled back</entry>
    </row>
    <row>
     <entry><structfield>blks_read</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of disk blocks read in this database</entry>
    </row>
    <row>
     <entry><structfield>blks_hit</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of times disk blocks were found already in the buffer
      cache, so that a read was not necessary (this only includes hits in the
      PostgreSQL buffer cache, not the operating system's file system cache)
     </entry>
    </row>
    <row>
     <entry><structfield>tup_returned</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of rows returned by queries in this database</entry>
    </row>
    <row>
     <entry><structfield>tup_fetched</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of rows fetched by queries in this database</entry>
    </row>
    <row>
     <entry><structfield>tup_inserted</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of rows inserted by queries in this database</entry>
    </row>
    <row>
     <entry><structfield>tup_updated</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of rows updated by queries in this database</entry>
    </row>
    <row>
     <entry><structfield>tup_deleted</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of rows deleted by queries in this database</entry>
    </row>
    <row>
     <entry><structfield>conflicts</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of queries canceled due to conflicts with recovery
      in this database. (Conflicts occur only on standby servers; see
      <xref linkend="pg-stat-database-conflicts-view"/> for details.)
     </entry>
    </row>
    <row>
     <entry><structfield>temp_files</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of temporary files created by queries in this database.
      All temporary files are counted, regardless of why the temporary file
      was created (e.g., sorting or hashing), and regardless of the
      <xref linkend="guc-log-temp-files"/> setting.
     </entry>
    </row>
    <row>
     <entry><structfield>temp_bytes</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Total amount of data written to temporary files by queries in
      this database. All temporary files are counted, regardless of why
      the temporary file was created, and
      regardless of the <xref linkend="guc-log-temp-files"/> setting.
     </entry>
    </row>
    <row>
     <entry><structfield>deadlocks</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of deadlocks detected in this database</entry>
    </row>
    <row>
     <entry><structfield>blk_read_time</structfield></entry>
     <entry><type>double precision</type></entry>
     <entry>Time spent reading data file blocks by backends in this database,
      in milliseconds</entry>
    </row>
    <row>
     <entry><structfield>blk_write_time</structfield></entry>
     <entry><type>double precision</type></entry>
     <entry>Time spent writing data file blocks by backends in this database,
      in milliseconds</entry>
    </row>
    <row>
     <entry><structfield>stats_reset</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Time at which these statistics were last reset</entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_database</structname> view will contain one row
   for each database in the cluster, showing database-wide statistics.
  </para>

  <table id="pg-stat-database-conflicts-view" xreflabel="pg_stat_database_conflicts">
   <title><structname>pg_stat_database_conflicts</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>datid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of a database</entry>
    </row>
    <row>
     <entry><structfield>datname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of this database</entry>
    </row>
    <row>
     <entry><structfield>confl_tablespace</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of queries in this database that have been canceled due to
      dropped tablespaces</entry>
    </row>
    <row>
     <entry><structfield>confl_lock</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of queries in this database that have been canceled due to
      lock timeouts</entry>
    </row>
    <row>
     <entry><structfield>confl_snapshot</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of queries in this database that have been canceled due to
      old snapshots</entry>
    </row>
    <row>
     <entry><structfield>confl_bufferpin</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of queries in this database that have been canceled due to
      pinned buffers</entry>
    </row>
    <row>
     <entry><structfield>confl_deadlock</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of queries in this database that have been canceled due to
      deadlocks</entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_database_conflicts</structname> view will contain
   one row per database, showing database-wide statistics about
   query cancels occurring due to conflicts with recovery on standby servers.
   This view will only contain information on standby servers, since
   conflicts do not occur on master servers.
  </para>

  <table id="pg-stat-all-tables-view" xreflabel="pg_stat_all_tables">
   <title><structname>pg_stat_all_tables</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>relid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of a table</entry>
    </row>
    <row>
     <entry><structfield>schemaname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the schema that this table is in</entry>
    </row>
    <row>
     <entry><structfield>relname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of this table</entry>
    </row>
    <row>
     <entry><structfield>seq_scan</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of sequential scans initiated on this table</entry>
    </row>
    <row>
     <entry><structfield>seq_tup_read</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of live rows fetched by sequential scans</entry>
    </row>
    <row>
     <entry><structfield>idx_scan</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of index scans initiated on this table</entry>
    </row>
    <row>
     <entry><structfield>idx_tup_fetch</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of live rows fetched by index scans</entry>
    </row>
    <row>
     <entry><structfield>n_tup_ins</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of rows inserted</entry>
    </row>
    <row>
     <entry><structfield>n_tup_upd</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of rows updated (includes HOT updated rows)</entry>
    </row>
    <row>
     <entry><structfield>n_tup_del</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of rows deleted</entry>
    </row>
    <row>
     <entry><structfield>n_tup_hot_upd</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of rows HOT updated (i.e., with no separate index
      update required)</entry>
    </row>
    <row>
     <entry><structfield>n_live_tup</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Estimated number of live rows</entry>
    </row>
    <row>
     <entry><structfield>n_dead_tup</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Estimated number of dead rows</entry>
    </row>
    <row>
     <entry><structfield>n_mod_since_analyze</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Estimated number of rows modified since this table was last analyzed</entry>
    </row>
    <row>
     <entry><structfield>last_vacuum</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Last time at which this table was manually vacuumed
      (not counting <command>VACUUM FULL</command>)</entry>
    </row>
    <row>
     <entry><structfield>last_autovacuum</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Last time at which this table was vacuumed by the autovacuum
      daemon</entry>
    </row>
    <row>
     <entry><structfield>last_analyze</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Last time at which this table was manually analyzed</entry>
    </row>
    <row>
     <entry><structfield>last_autoanalyze</structfield></entry>
     <entry><type>timestamp with time zone</type></entry>
     <entry>Last time at which this table was analyzed by the autovacuum
      daemon</entry>
    </row>
    <row>
     <entry><structfield>vacuum_count</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of times this table has been manually vacuumed
      (not counting <command>VACUUM FULL</command>)</entry>
    </row>
    <row>
     <entry><structfield>autovacuum_count</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of times this table has been vacuumed by the autovacuum
      daemon</entry>
    </row>
    <row>
     <entry><structfield>analyze_count</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of times this table has been manually analyzed</entry>
    </row>
    <row>
     <entry><structfield>autoanalyze_count</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of times this table has been analyzed by the autovacuum
      daemon</entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_all_tables</structname> view will contain
   one row for each table in the current database (including TOAST
   tables), showing statistics about accesses to that specific table. The
   <structname>pg_stat_user_tables</structname> and
   <structname>pg_stat_sys_tables</structname> views
   contain the same information,
   but filtered to only show user and system tables respectively.
  </para>

  <table id="pg-stat-all-indexes-view" xreflabel="pg_stat_all_indexes">
   <title><structname>pg_stat_all_indexes</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>relid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of the table for this index</entry>
    </row>
    <row>
     <entry><structfield>indexrelid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of this index</entry>
    </row>
    <row>
     <entry><structfield>schemaname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the schema this index is in</entry>
    </row>
    <row>
     <entry><structfield>relname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the table for this index</entry>
    </row>
    <row>
     <entry><structfield>indexrelname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of this index</entry>
    </row>
    <row>
     <entry><structfield>idx_scan</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of index scans initiated on this index</entry>
    </row>
    <row>
     <entry><structfield>idx_tup_read</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of index entries returned by scans on this index</entry>
    </row>
    <row>
     <entry><structfield>idx_tup_fetch</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of live table rows fetched by simple index scans using this
      index</entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_all_indexes</structname> view will contain
   one row for each index in the current database,
   showing statistics about accesses to that specific index. The
   <structname>pg_stat_user_indexes</structname> and
   <structname>pg_stat_sys_indexes</structname> views
   contain the same information,
   but filtered to only show user and system indexes respectively.
  </para>

  <para>
   Indexes can be used by simple index scans, <quote>bitmap</quote> index scans,
   and the optimizer.  In a bitmap scan
   the output of several indexes can be combined via AND or OR rules,
   so it is difficult to associate individual heap row fetches
   with specific indexes when a bitmap scan is used.  Therefore, a bitmap
   scan increments the
   <structname>pg_stat_all_indexes</structname>.<structfield>idx_tup_read</structfield>
   count(s) for the index(es) it uses, and it increments the
   <structname>pg_stat_all_tables</structname>.<structfield>idx_tup_fetch</structfield>
   count for the table, but it does not affect
   <structname>pg_stat_all_indexes</structname>.<structfield>idx_tup_fetch</structfield>.
   The optimizer also accesses indexes to check for supplied constants
   whose values are outside the recorded range of the optimizer statistics
   because the optimizer statistics might be stale.
  </para>

  <note>
   <para>
    The <structfield>idx_tup_read</structfield> and <structfield>idx_tup_fetch</structfield> counts
    can be different even without any use of bitmap scans,
    because <structfield>idx_tup_read</structfield> counts
    index entries retrieved from the index while <structfield>idx_tup_fetch</structfield>
    counts live rows fetched from the table.  The latter will be less if any
    dead or not-yet-committed rows are fetched using the index, or if any
    heap fetches are avoided by means of an index-only scan.
   </para>
  </note>

  <table id="pg-statio-all-tables-view" xreflabel="pg_statio_all_tables">
   <title><structname>pg_statio_all_tables</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>relid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of a table</entry>
    </row>
    <row>
     <entry><structfield>schemaname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the schema that this table is in</entry>
    </row>
    <row>
     <entry><structfield>relname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of this table</entry>
    </row>
    <row>
     <entry><structfield>heap_blks_read</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of disk blocks read from this table</entry>
    </row>
    <row>
     <entry><structfield>heap_blks_hit</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of buffer hits in this table</entry>
    </row>
    <row>
     <entry><structfield>idx_blks_read</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of disk blocks read from all indexes on this table</entry>
    </row>
    <row>
     <entry><structfield>idx_blks_hit</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of buffer hits in all indexes on this table</entry>
    </row>
    <row>
     <entry><structfield>toast_blks_read</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of disk blocks read from this table's TOAST table (if any)</entry>
    </row>
    <row>
     <entry><structfield>toast_blks_hit</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of buffer hits in this table's TOAST table (if any)</entry>
    </row>
    <row>
     <entry><structfield>tidx_blks_read</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of disk blocks read from this table's TOAST table indexes (if any)</entry>
    </row>
    <row>
     <entry><structfield>tidx_blks_hit</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of buffer hits in this table's TOAST table indexes (if any)</entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_statio_all_tables</structname> view will contain
   one row for each table in the current database (including TOAST
   tables), showing statistics about I/O on that specific table. The
   <structname>pg_statio_user_tables</structname> and
   <structname>pg_statio_sys_tables</structname> views
   contain the same information,
   but filtered to only show user and system tables respectively.
  </para>

  <table id="pg-statio-all-indexes-view" xreflabel="pg_statio_all_indexes">
   <title><structname>pg_statio_all_indexes</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>relid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of the table for this index</entry>
    </row>
    <row>
     <entry><structfield>indexrelid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of this index</entry>
    </row>
    <row>
     <entry><structfield>schemaname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the schema this index is in</entry>
    </row>
    <row>
     <entry><structfield>relname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the table for this index</entry>
    </row>
    <row>
     <entry><structfield>indexrelname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of this index</entry>
    </row>
    <row>
     <entry><structfield>idx_blks_read</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of disk blocks read from this index</entry>
    </row>
    <row>
     <entry><structfield>idx_blks_hit</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of buffer hits in this index</entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_statio_all_indexes</structname> view will contain
   one row for each index in the current database,
   showing statistics about I/O on that specific index. The
   <structname>pg_statio_user_indexes</structname> and
   <structname>pg_statio_sys_indexes</structname> views
   contain the same information,
   but filtered to only show user and system indexes respectively.
  </para>

  <table id="pg-statio-all-sequences-view" xreflabel="pg_statio_all_sequences">
   <title><structname>pg_statio_all_sequences</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>relid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of a sequence</entry>
    </row>
    <row>
     <entry><structfield>schemaname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the schema this sequence is in</entry>
    </row>
    <row>
     <entry><structfield>relname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of this sequence</entry>
    </row>
    <row>
     <entry><structfield>blks_read</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of disk blocks read from this sequence</entry>
    </row>
    <row>
     <entry><structfield>blks_hit</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of buffer hits in this sequence</entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_statio_all_sequences</structname> view will contain
   one row for each sequence in the current database,
   showing statistics about I/O on that specific sequence.
  </para>

  <table id="pg-stat-user-functions-view" xreflabel="pg_stat_user_functions">
   <title><structname>pg_stat_user_functions</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>funcid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of a function</entry>
    </row>
    <row>
     <entry><structfield>schemaname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the schema this function is in</entry>
    </row>
    <row>
     <entry><structfield>funcname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of this function</entry>
    </row>
    <row>
     <entry><structfield>calls</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>Number of times this function has been called</entry>
    </row>
    <row>
     <entry><structfield>total_time</structfield></entry>
     <entry><type>double precision</type></entry>
     <entry>Total time spent in this function and all other functions
     called by it, in milliseconds</entry>
    </row>
    <row>
     <entry><structfield>self_time</structfield></entry>
     <entry><type>double precision</type></entry>
     <entry>Total time spent in this function itself, not including
     other functions called by it, in milliseconds</entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_user_functions</structname> view will contain
   one row for each tracked function, showing statistics about executions of
   that function.  The <xref linkend="guc-track-functions"/> parameter
   controls exactly which functions are tracked.
  </para>

 </sect2>

 <sect2 id="monitoring-stats-functions">
  <title>Statistics Functions</title>

  <para>
   Other ways of looking at the statistics can be set up by writing
   queries that use the same underlying statistics access functions used by
   the standard views shown above.  For details such as the functions' names,
   consult the definitions of the standard views.  (For example, in
   <application>psql</application> you could issue <literal>\d+ pg_stat_activity</literal>.)
   The access functions for per-database statistics take a database OID as an
   argument to identify which database to report on.
   The per-table and per-index functions take a table or index OID.
   The functions for per-function statistics take a function OID.
   Note that only tables, indexes, and functions in the current database
   can be seen with these functions.
  </para>

  <para>
   Additional functions related to statistics collection are listed in <xref
   linkend="monitoring-stats-funcs-table"/>.
  </para>

  <table id="monitoring-stats-funcs-table">
   <title>Additional Statistics Functions</title>

   <tgroup cols="3">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>

     <row>
       <!-- See also the entry for this in func.sgml -->
      <entry><literal><function>pg_backend_pid()</function></literal></entry>
      <entry><type>integer</type></entry>
      <entry>
       Process ID of the server process handling the current session
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_activity</function>(<type>integer</type>)</literal><indexterm><primary>pg_stat_get_activity</primary></indexterm></entry>
      <entry><type>setof record</type></entry>
      <entry>
       Returns a record of information about the backend with the specified PID, or
       one record for each active backend in the system if <symbol>NULL</symbol> is
       specified. The fields returned are a subset of those in the
       <structname>pg_stat_activity</structname> view.
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_snapshot_timestamp()</function></literal><indexterm><primary>pg_stat_get_snapshot_timestamp</primary></indexterm></entry>
      <entry><type>timestamp with time zone</type></entry>
      <entry>
       Returns the timestamp of the current statistics snapshot
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_clear_snapshot()</function></literal><indexterm><primary>pg_stat_clear_snapshot</primary></indexterm></entry>
      <entry><type>void</type></entry>
      <entry>
       Discard the current statistics snapshot
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_reset()</function></literal><indexterm><primary>pg_stat_reset</primary></indexterm></entry>
      <entry><type>void</type></entry>
      <entry>
       Reset all statistics counters for the current database to zero
       (requires superuser privileges by default, but EXECUTE for this
       function can be granted to others.)
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_reset_shared</function>(text)</literal><indexterm><primary>pg_stat_reset_shared</primary></indexterm></entry>
      <entry><type>void</type></entry>
      <entry>
       Reset some cluster-wide statistics counters to zero, depending on the
       argument (requires superuser privileges by default, but EXECUTE for
       this function can be granted to others).
       Calling <literal>pg_stat_reset_shared('bgwriter')</literal> will zero all the
       counters shown in the <structname>pg_stat_bgwriter</structname> view.
       Calling <literal>pg_stat_reset_shared('archiver')</literal> will zero all the
       counters shown in the <structname>pg_stat_archiver</structname> view.
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_reset_single_table_counters</function>(oid)</literal><indexterm><primary>pg_stat_reset_single_table_counters</primary></indexterm></entry>
      <entry><type>void</type></entry>
      <entry>
       Reset statistics for a single table or index in the current database to
       zero (requires superuser privileges by default, but EXECUTE for this
       function can be granted to others)
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_reset_single_function_counters</function>(oid)</literal><indexterm><primary>pg_stat_reset_single_function_counters</primary></indexterm></entry>
      <entry><type>void</type></entry>
      <entry>
       Reset statistics for a single function in the current database to
       zero (requires superuser privileges by default, but EXECUTE for this
       function can be granted to others)
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   <function>pg_stat_get_activity</function>, the underlying function of
   the <structname>pg_stat_activity</structname> view, returns a set of records
   containing all the available information about each backend process.
   Sometimes it may be more convenient to obtain just a subset of this
   information.  In such cases, an older set of per-backend statistics
   access functions can be used; these are shown in <xref
   linkend="monitoring-stats-backend-funcs-table"/>.
   These access functions use a backend ID number, which ranges from one
   to the number of currently active backends.
   The function <function>pg_stat_get_backend_idset</function> provides a
   convenient way to generate one row for each active backend for
   invoking these functions.  For example, to show the <acronym>PID</acronym>s and
   current queries of all backends:

<programlisting>
SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
       pg_stat_get_backend_activity(s.backendid) AS query
    FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS s;
</programlisting>
  </para>

  <table id="monitoring-stats-backend-funcs-table">
   <title>Per-Backend Statistics Functions</title>

   <tgroup cols="3">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>

     <row>
      <entry><literal><function>pg_stat_get_backend_idset()</function></literal></entry>
      <entry><type>setof integer</type></entry>
      <entry>Set of currently active backend ID numbers (from 1 to the
       number of active backends)</entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_activity(integer)</function></literal></entry>
      <entry><type>text</type></entry>
      <entry>Text of this backend's most recent query</entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_activity_start(integer)</function></literal></entry>
      <entry><type>timestamp with time zone</type></entry>
      <entry>Time when the most recent query was started</entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_client_addr(integer)</function></literal></entry>
      <entry><type>inet</type></entry>
      <entry>IP address of the client connected to this backend</entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_client_port(integer)</function></literal></entry>
      <entry><type>integer</type></entry>
      <entry>TCP port number that the client is using for communication</entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_dbid(integer)</function></literal></entry>
      <entry><type>oid</type></entry>
      <entry>OID of the database this backend is connected to</entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_pid(integer)</function></literal></entry>
      <entry><type>integer</type></entry>
      <entry>Process ID of this backend</entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_start(integer)</function></literal></entry>
      <entry><type>timestamp with time zone</type></entry>
      <entry>Time when this process was started</entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_userid(integer)</function></literal></entry>
      <entry><type>oid</type></entry>
      <entry>OID of the user logged into this backend</entry>
     </row>

      <row>
       <entry><literal><function>pg_stat_get_backend_wait_event_type(integer)</function></literal></entry>
       <entry><type>text</type></entry>
        <entry>Wait event type name if backend is currently waiting, otherwise NULL.
        See <xref linkend="wait-event-table"/> for details.
        </entry>
      </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_wait_event(integer)</function></literal></entry>
      <entry><type>text</type></entry>
       <entry>Wait event name if backend is currently waiting, otherwise NULL.
       See <xref linkend="wait-event-table"/> for details.
       </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_xact_start(integer)</function></literal></entry>
      <entry><type>timestamp with time zone</type></entry>
      <entry>Time when the current transaction was started</entry>
     </row>

    </tbody>
   </tgroup>
  </table>

 </sect2>
 </sect1>

 <sect1 id="monitoring-locks">
  <title>Viewing Locks</title>

  <indexterm zone="monitoring-locks">
   <primary>lock</primary>
   <secondary>monitoring</secondary>
  </indexterm>

  <para>
   Another useful tool for monitoring database activity is the
   <structname>pg_locks</structname> system table.  It allows the
   database administrator to view information about the outstanding
   locks in the lock manager. For example, this capability can be used
   to:

   <itemizedlist>
    <listitem>
     <para>
      View all the locks currently outstanding, all the locks on
      relations in a particular database, all the locks on a
      particular relation, or all the locks held by a particular
      <productname>PostgreSQL</productname> session.
     </para>
    </listitem>

    <listitem>
     <para>
      Determine the relation in the current database with the most
      ungranted locks (which might be a source of contention among
      database clients).
     </para>
    </listitem>

    <listitem>
     <para>
      Determine the effect of lock contention on overall database
      performance, as well as the extent to which contention varies
      with overall database traffic.
     </para>
    </listitem>
   </itemizedlist>

   Details of the <structname>pg_locks</structname> view appear in
   <xref linkend="view-pg-locks"/>.
   For more information on locking and managing concurrency with
   <productname>PostgreSQL</productname>, refer to <xref linkend="mvcc"/>.
  </para>
 </sect1>

 <sect1 id="progress-reporting">
  <title>Progress Reporting</title>

  <para>
   <productname>PostgreSQL</productname> has the ability to report the progress of
   certain commands during command execution.  Currently, the only command
   which supports progress reporting is <command>VACUUM</command>.  This may be
   expanded in the future.
  </para>

 <sect2 id="vacuum-progress-reporting">
  <title>VACUUM Progress Reporting</title>

  <para>
   Whenever <command>VACUUM</command> is running, the
   <structname>pg_stat_progress_vacuum</structname> view will contain
   one row for each backend (including autovacuum worker processes) that is
   currently vacuuming.  The tables below describe the information
   that will be reported and provide information about how to interpret it.
   Progress reporting is not currently supported for <command>VACUUM FULL</command>
   and backends running <command>VACUUM FULL</command> will not be listed in this
   view.
  </para>

  <table id="pg-stat-progress-vacuum-view" xreflabel="pg_stat_progress_vacuum">
   <title><structname>pg_stat_progress_vacuum</structname> View</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><structfield>pid</structfield></entry>
     <entry><type>integer</type></entry>
     <entry>Process ID of backend.</entry>
    </row>
    <row>
     <entry><structfield>datid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of the database to which this backend is connected.</entry>
    </row>
    <row>
     <entry><structfield>datname</structfield></entry>
     <entry><type>name</type></entry>
     <entry>Name of the database to which this backend is connected.</entry>
    </row>
    <row>
     <entry><structfield>relid</structfield></entry>
     <entry><type>oid</type></entry>
     <entry>OID of the table being vacuumed.</entry>
    </row>
    <row>
     <entry><structfield>phase</structfield></entry>
     <entry><type>text</type></entry>
     <entry>
       Current processing phase of vacuum.  See <xref linkend='vacuum-phases'/>.
     </entry>
    </row>
    <row>
     <entry><structfield>heap_blks_total</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>
       Total number of heap blocks in the table.  This number is reported
       as of the beginning of the scan; blocks added later will not be (and
       need not be) visited by this <command>VACUUM</command>.
     </entry>
    </row>
    <row>
     <entry><structfield>heap_blks_scanned</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>
       Number of heap blocks scanned.  Because the
       <link linkend="storage-vm">visibility map</link> is used to optimize scans,
       some blocks will be skipped without inspection; skipped blocks are
       included in this total, so that this number will eventually become
       equal to <structfield>heap_blks_total</structfield> when the vacuum is complete.
       This counter only advances when the phase is <literal>scanning heap</literal>.
     </entry>
    </row>
    <row>
     <entry><structfield>heap_blks_vacuumed</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>
       Number of heap blocks vacuumed.  Unless the table has no indexes, this
       counter only advances when the phase is <literal>vacuuming heap</literal>.
       Blocks that contain no dead tuples are skipped, so the counter may
       sometimes skip forward in large increments.
     </entry>
    </row>
    <row>
     <entry><structfield>index_vacuum_count</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>
       Number of completed index vacuum cycles.
     </entry>
    </row>
    <row>
     <entry><structfield>max_dead_tuples</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>
      Number of dead tuples that we can store before needing to perform
      an index vacuum cycle, based on
      <xref linkend="guc-maintenance-work-mem"/>.
     </entry>
    </row>
    <row>
     <entry><structfield>num_dead_tuples</structfield></entry>
     <entry><type>bigint</type></entry>
     <entry>
       Number of dead tuples collected since the last index vacuum cycle.
     </entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <table id="vacuum-phases">
   <title>VACUUM phases</title>
   <tgroup cols="2">
    <thead>
    <row>
      <entry>Phase</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><literal>initializing</literal></entry>
     <entry>
       <command>VACUUM</command> is preparing to begin scanning the heap.  This
       phase is expected to be very brief.
     </entry>
    </row>
    <row>
     <entry><literal>scanning heap</literal></entry>
     <entry>
       <command>VACUUM</command> is currently scanning the heap.  It will prune and
       defragment each page if required, and possibly perform freezing
       activity.  The <structfield>heap_blks_scanned</structfield> column can be used
       to monitor the progress of the scan.
     </entry>
    </row>
    <row>
     <entry><literal>vacuuming indexes</literal></entry>
     <entry>
       <command>VACUUM</command> is currently vacuuming the indexes.  If a table has
       any indexes, this will happen at least once per vacuum, after the heap
       has been completely scanned.  It may happen multiple times per vacuum
       if <xref linkend="guc-maintenance-work-mem"/> is insufficient to
       store the number of dead tuples found.
     </entry>
    </row>
    <row>
     <entry><literal>vacuuming heap</literal></entry>
     <entry>
       <command>VACUUM</command> is currently vacuuming the heap.  Vacuuming the heap
       is distinct from scanning the heap, and occurs after each instance of
       vacuuming indexes.  If <structfield>heap_blks_scanned</structfield> is less than
       <structfield>heap_blks_total</structfield>, the system will return to scanning
       the heap after this phase is completed; otherwise, it will begin
       cleaning up indexes after this phase is completed.
     </entry>
    </row>
    <row>
     <entry><literal>cleaning up indexes</literal></entry>
     <entry>
       <command>VACUUM</command> is currently cleaning up indexes.  This occurs after
       the heap has been completely scanned and all vacuuming of the indexes
       and the heap has been completed.
     </entry>
    </row>
    <row>
     <entry><literal>truncating heap</literal></entry>
     <entry>
       <command>VACUUM</command> is currently truncating the heap so as to return
       empty pages at the end of the relation to the operating system.  This
       occurs after cleaning up indexes.
     </entry>
    </row>
    <row>
     <entry><literal>performing final cleanup</literal></entry>
     <entry>
       <command>VACUUM</command> is performing final cleanup.  During this phase,
       <command>VACUUM</command> will vacuum the free space map, update statistics
       in <literal>pg_class</literal>, and report statistics to the statistics
       collector.  When this phase is completed, <command>VACUUM</command> will end.
     </entry>
    </row>
   </tbody>
   </tgroup>
  </table>

 </sect2>
 </sect1>

 <sect1 id="dynamic-trace">
  <title>Dynamic Tracing</title>

 <indexterm zone="dynamic-trace">
  <primary>DTrace</primary>
 </indexterm>

  <para>
   <productname>PostgreSQL</productname> provides facilities to support
   dynamic tracing of the database server. This allows an external
   utility to be called at specific points in the code and thereby trace
   execution.
  </para>

  <para>
   A number of probes or trace points are already inserted into the source
   code. These probes are intended to be used by database developers and
   administrators. By default the probes are not compiled into
   <productname>PostgreSQL</productname>; the user needs to explicitly tell
   the configure script to make the probes available.
  </para>

  <para>
   Currently, the
   <ulink url="https://en.wikipedia.org/wiki/DTrace">DTrace</ulink>
   utility is supported, which, at the time of this writing, is available
   on Solaris, macOS, FreeBSD, NetBSD, and Oracle Linux.  The
   <ulink url="http://sourceware.org/systemtap/">SystemTap</ulink> project
   for Linux provides a DTrace equivalent and can also be used.  Supporting other dynamic
   tracing utilities is theoretically possible by changing the definitions for
   the macros in <filename>src/include/utils/probes.h</filename>.
  </para>

  <sect2 id="compiling-for-trace">
   <title>Compiling for Dynamic Tracing</title>

  <para>
   By default, probes are not available, so you will need to
   explicitly tell the configure script to make the probes available
   in <productname>PostgreSQL</productname>. To include DTrace support
   specify <option>--enable-dtrace</option> to configure.  See <xref
   linkend="install-procedure"/> for further information.
  </para>
  </sect2>

  <sect2 id="trace-points">
   <title>Built-in Probes</title>

  <para>
   A number of standard probes are provided in the source code,
   as shown in <xref linkend="dtrace-probe-point-table"/>;
   <xref linkend="typedefs-table"/>
   shows the types used in the probes.  More probes can certainly be
   added to enhance <productname>PostgreSQL</productname>'s observability.
  </para>

 <table id="dtrace-probe-point-table">
  <title>Built-in DTrace Probes</title>
  <tgroup cols="3">
   <thead>
    <row>
     <entry>Name</entry>
     <entry>Parameters</entry>
     <entry>Description</entry>
    </row>
   </thead>

   <tbody>

    <row>
     <entry><literal>transaction-start</literal></entry>
     <entry><literal>(LocalTransactionId)</literal></entry>
     <entry>Probe that fires at the start of a new transaction.
      arg0 is the transaction ID.</entry>
    </row>
    <row>
     <entry><literal>transaction-commit</literal></entry>
     <entry><literal>(LocalTransactionId)</literal></entry>
     <entry>Probe that fires when a transaction completes successfully.
      arg0 is the transaction ID.</entry>
    </row>
    <row>
     <entry><literal>transaction-abort</literal></entry>
     <entry><literal>(LocalTransactionId)</literal></entry>
     <entry>Probe that fires when a transaction completes unsuccessfully.
      arg0 is the transaction ID.</entry>
    </row>
    <row>
     <entry><literal>query-start</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the processing of a query is started.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-done</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the processing of a query is complete.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-parse-start</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the parsing of a query is started.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-parse-done</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the parsing of a query is complete.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-rewrite-start</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the rewriting of a query is started.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-rewrite-done</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the rewriting of a query is complete.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-plan-start</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the planning of a query is started.</entry>
    </row>
    <row>
     <entry><literal>query-plan-done</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the planning of a query is complete.</entry>
    </row>
    <row>
     <entry><literal>query-execute-start</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the execution of a query is started.</entry>
    </row>
    <row>
     <entry><literal>query-execute-done</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the execution of a query is complete.</entry>
    </row>
    <row>
     <entry><literal>statement-status</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires anytime the server process updates its
      <structname>pg_stat_activity</structname>.<structfield>status</structfield>.
      arg0 is the new status string.</entry>
    </row>
    <row>
     <entry><literal>checkpoint-start</literal></entry>
     <entry><literal>(int)</literal></entry>
     <entry>Probe that fires when a checkpoint is started.
      arg0 holds the bitwise flags used to distinguish different checkpoint
      types, such as shutdown, immediate or force.</entry>
    </row>
    <row>
     <entry><literal>checkpoint-done</literal></entry>
     <entry><literal>(int, int, int, int, int)</literal></entry>
     <entry>Probe that fires when a checkpoint is complete.
      (The probes listed next fire in sequence during checkpoint processing.)
      arg0 is the number of buffers written. arg1 is the total number of
      buffers. arg2, arg3 and arg4 contain the number of WAL files added,
      removed and recycled respectively.</entry>
    </row>
    <row>
     <entry><literal>clog-checkpoint-start</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the CLOG portion of a checkpoint is started.
      arg0 is true for normal checkpoint, false for shutdown
      checkpoint.</entry>
    </row>
    <row>
     <entry><literal>clog-checkpoint-done</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the CLOG portion of a checkpoint is
      complete. arg0 has the same meaning as for <literal>clog-checkpoint-start</literal>.</entry>
    </row>
    <row>
     <entry><literal>subtrans-checkpoint-start</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the SUBTRANS portion of a checkpoint is
      started.
      arg0 is true for normal checkpoint, false for shutdown
      checkpoint.</entry>
    </row>
    <row>
     <entry><literal>subtrans-checkpoint-done</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the SUBTRANS portion of a checkpoint is
      complete. arg0 has the same meaning as for
      <literal>subtrans-checkpoint-start</literal>.</entry>
    </row>
    <row>
     <entry><literal>multixact-checkpoint-start</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the MultiXact portion of a checkpoint is
      started.
      arg0 is true for normal checkpoint, false for shutdown
      checkpoint.</entry>
    </row>
    <row>
     <entry><literal>multixact-checkpoint-done</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the MultiXact portion of a checkpoint is
      complete. arg0 has the same meaning as for
      <literal>multixact-checkpoint-start</literal>.</entry>
    </row>
    <row>
     <entry><literal>buffer-checkpoint-start</literal></entry>
     <entry><literal>(int)</literal></entry>
     <entry>Probe that fires when the buffer-writing portion of a checkpoint
      is started.
      arg0 holds the bitwise flags used to distinguish different checkpoint
      types, such as shutdown, immediate or force.</entry>
    </row>
    <row>
     <entry><literal>buffer-sync-start</literal></entry>
     <entry><literal>(int, int)</literal></entry>
     <entry>Probe that fires when we begin to write dirty buffers during
      checkpoint (after identifying which buffers must be written).
      arg0 is the total number of buffers.
      arg1 is the number that are currently dirty and need to be written.</entry>
    </row>
    <row>
     <entry><literal>buffer-sync-written</literal></entry>
     <entry><literal>(int)</literal></entry>
     <entry>Probe that fires after each buffer is written during checkpoint.
      arg0 is the ID number of the buffer.</entry>
    </row>
    <row>
     <entry><literal>buffer-sync-done</literal></entry>
     <entry><literal>(int, int, int)</literal></entry>
     <entry>Probe that fires when all dirty buffers have been written.
      arg0 is the total number of buffers.
      arg1 is the number of buffers actually written by the checkpoint process.
      arg2 is the number that were expected to be written (arg1 of
      <literal>buffer-sync-start</literal>); any difference reflects other processes flushing
      buffers during the checkpoint.</entry>
    </row>
    <row>
     <entry><literal>buffer-checkpoint-sync-start</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires after dirty buffers have been written to the
      kernel, and before starting to issue fsync requests.</entry>
    </row>
    <row>
     <entry><literal>buffer-checkpoint-done</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when syncing of buffers to disk is
      complete.</entry>
    </row>
    <row>
     <entry><literal>twophase-checkpoint-start</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the two-phase portion of a checkpoint is
      started.</entry>
    </row>
    <row>
     <entry><literal>twophase-checkpoint-done</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the two-phase portion of a checkpoint is
      complete.</entry>
    </row>
    <row>
     <entry><literal>buffer-read-start</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool)</literal></entry>
     <entry>Probe that fires when a buffer read is started.
      arg0 and arg1 contain the fork and block numbers of the page (but
      arg1 will be -1 if this is a relation extension request).
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.
      arg6 is true for a relation extension request, false for normal
      read.</entry>
    </row>
    <row>
     <entry><literal>buffer-read-done</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool, bool)</literal></entry>
     <entry>Probe that fires when a buffer read is complete.
      arg0 and arg1 contain the fork and block numbers of the page (if this
      is a relation extension request, arg1 now contains the block number
      of the newly added block).
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.
      arg6 is true for a relation extension request, false for normal
      read.
      arg7 is true if the buffer was found in the pool, false if not.</entry>
    </row>
    <row>
     <entry><literal>buffer-flush-start</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid)</literal></entry>
     <entry>Probe that fires before issuing any write request for a shared
      buffer.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.</entry>
    </row>
    <row>
     <entry><literal>buffer-flush-done</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid)</literal></entry>
     <entry>Probe that fires when a write request is complete.  (Note
      that this just reflects the time to pass the data to the kernel;
      it's typically not actually been written to disk yet.)
      The arguments are the same as for <literal>buffer-flush-start</literal>.</entry>
    </row>
    <row>
     <entry><literal>buffer-write-dirty-start</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid)</literal></entry>
     <entry>Probe that fires when a server process begins to write a dirty
      buffer.  (If this happens often, it implies that
      <xref linkend="guc-shared-buffers"/> is too
      small or the background writer control parameters need adjustment.)
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.</entry>
    </row>
    <row>
     <entry><literal>buffer-write-dirty-done</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid)</literal></entry>
     <entry>Probe that fires when a dirty-buffer write is complete.
      The arguments are the same as for <literal>buffer-write-dirty-start</literal>.</entry>
    </row>
    <row>
     <entry><literal>wal-buffer-write-dirty-start</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when a server process begins to write a
      dirty WAL buffer because no more WAL buffer space is available.
      (If this happens often, it implies that
      <xref linkend="guc-wal-buffers"/> is too small.)</entry>
    </row>
    <row>
     <entry><literal>wal-buffer-write-dirty-done</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when a dirty WAL buffer write is complete.</entry>
    </row>
    <row>
     <entry><literal>wal-insert</literal></entry>
     <entry><literal>(unsigned char, unsigned char)</literal></entry>
     <entry>Probe that fires when a WAL record is inserted.
      arg0 is the resource manager (rmid) for the record.
      arg1 contains the info flags.</entry>
    </row>
    <row>
     <entry><literal>wal-switch</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when a WAL segment switch is requested.</entry>
    </row>
    <row>
     <entry><literal>smgr-md-read-start</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</literal></entry>
     <entry>Probe that fires when beginning to read a block from a relation.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.</entry>
    </row>
    <row>
     <entry><literal>smgr-md-read-done</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</literal></entry>
     <entry>Probe that fires when a block read is complete.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.
      arg6 is the number of bytes actually read, while arg7 is the number
      requested (if these are different it indicates trouble).</entry>
    </row>
    <row>
     <entry><literal>smgr-md-write-start</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</literal></entry>
     <entry>Probe that fires when beginning to write a block to a relation.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.</entry>
    </row>
    <row>
     <entry><literal>smgr-md-write-done</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</literal></entry>
     <entry>Probe that fires when a block write is complete.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.
      arg6 is the number of bytes actually written, while arg7 is the number
      requested (if these are different it indicates trouble).</entry>
    </row>
    <row>
     <entry><literal>sort-start</literal></entry>
     <entry><literal>(int, bool, int, int, bool, int)</literal></entry>
     <entry>Probe that fires when a sort operation is started.
      arg0 indicates heap, index or datum sort.
      arg1 is true for unique-value enforcement.
      arg2 is the number of key columns.
      arg3 is the number of kilobytes of work memory allowed.
      arg4 is true if random access to the sort result is required.
      arg5 indicates serial when <literal>0</literal>, parallel worker when
      <literal>1</literal>, or parallel leader when <literal>2</literal>.</entry>
    </row>
    <row>
     <entry><literal>sort-done</literal></entry>
     <entry><literal>(bool, long)</literal></entry>
     <entry>Probe that fires when a sort is complete.
      arg0 is true for external sort, false for internal sort.
      arg1 is the number of disk blocks used for an external sort,
      or kilobytes of memory used for an internal sort.</entry>
    </row>
    <row>
     <entry><literal>lwlock-acquire</literal></entry>
     <entry><literal>(char *, LWLockMode)</literal></entry>
     <entry>Probe that fires when an LWLock has been acquired.
      arg0 is the LWLock's tranche.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry><literal>lwlock-release</literal></entry>
     <entry><literal>(char *)</literal></entry>
     <entry>Probe that fires when an LWLock has been released (but note
      that any released waiters have not yet been awakened).
      arg0 is the LWLock's tranche.</entry>
    </row>
    <row>
     <entry><literal>lwlock-wait-start</literal></entry>
     <entry><literal>(char *, LWLockMode)</literal></entry>
     <entry>Probe that fires when an LWLock was not immediately available and
      a server process has begun to wait for the lock to become available.
      arg0 is the LWLock's tranche.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry><literal>lwlock-wait-done</literal></entry>
     <entry><literal>(char *, LWLockMode)</literal></entry>
     <entry>Probe that fires when a server process has been released from its
      wait for an LWLock (it does not actually have the lock yet).
      arg0 is the LWLock's tranche.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry><literal>lwlock-condacquire</literal></entry>
     <entry><literal>(char *, LWLockMode)</literal></entry>
     <entry>Probe that fires when an LWLock was successfully acquired when the
      caller specified no waiting.
      arg0 is the LWLock's tranche.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry><literal>lwlock-condacquire-fail</literal></entry>
     <entry><literal>(char *, LWLockMode)</literal></entry>
     <entry>Probe that fires when an LWLock was not successfully acquired when
      the caller specified no waiting.
      arg0 is the LWLock's tranche.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry><literal>lock-wait-start</literal></entry>
     <entry><literal>(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</literal></entry>
     <entry>Probe that fires when a request for a heavyweight lock (lmgr lock)
      has begun to wait because the lock is not available.
      arg0 through arg3 are the tag fields identifying the object being
      locked.  arg4 indicates the type of object being locked.
      arg5 indicates the lock type being requested.</entry>
    </row>
    <row>
     <entry><literal>lock-wait-done</literal></entry>
     <entry><literal>(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</literal></entry>
     <entry>Probe that fires when a request for a heavyweight lock (lmgr lock)
      has finished waiting (i.e., has acquired the lock).
      The arguments are the same as for <literal>lock-wait-start</literal>.</entry>
    </row>
    <row>
     <entry><literal>deadlock-found</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when a deadlock is found by the deadlock
      detector.</entry>
    </row>

   </tbody>
   </tgroup>
  </table>

 <table id="typedefs-table">
  <title>Defined Types Used in Probe Parameters</title>
  <tgroup cols="2">
   <thead>
    <row>
     <entry>Type</entry>
     <entry>Definition</entry>
    </row>
   </thead>

   <tbody>

    <row>
     <entry><type>LocalTransactionId</type></entry>
     <entry><type>unsigned int</type></entry>
    </row>
    <row>
     <entry><type>LWLockMode</type></entry>
     <entry><type>int</type></entry>
    </row>
    <row>
     <entry><type>LOCKMODE</type></entry>
     <entry><type>int</type></entry>
    </row>
    <row>
     <entry><type>BlockNumber</type></entry>
     <entry><type>unsigned int</type></entry>
    </row>
    <row>
     <entry><type>Oid</type></entry>
     <entry><type>unsigned int</type></entry>
    </row>
    <row>
     <entry><type>ForkNumber</type></entry>
     <entry><type>int</type></entry>
    </row>
    <row>
     <entry><type>bool</type></entry>
     <entry><type>char</type></entry>
    </row>

   </tbody>
   </tgroup>
  </table>


  </sect2>

  <sect2 id="using-trace-points">
   <title>Using Probes</title>

  <para>
   The example below shows a DTrace script for analyzing transaction
   counts in the system, as an alternative to snapshotting
   <structname>pg_stat_database</structname> before and after a performance test:
<programlisting>
#!/usr/sbin/dtrace -qs

postgresql$1:::transaction-start
{
      @start["Start"] = count();
      self->ts  = timestamp;
}

postgresql$1:::transaction-abort
{
      @abort["Abort"] = count();
}

postgresql$1:::transaction-commit
/self->ts/
{
      @commit["Commit"] = count();
      @time["Total time (ns)"] = sum(timestamp - self->ts);
      self->ts=0;
}
</programlisting>
   When executed, the example D script gives output such as:
<screen>
# ./txn_count.d `pgrep -n postgres` or ./txn_count.d &lt;PID&gt;
^C

Start                                          71
Commit                                         70
Total time (ns)                        2312105013
</screen>
  </para>

  <note>
   <para>
    SystemTap uses a different notation for trace scripts than DTrace does,
    even though the underlying trace points are compatible.  One point worth
    noting is that at this writing, SystemTap scripts must reference probe
    names using double underscores in place of hyphens.  This is expected to
    be fixed in future SystemTap releases.
   </para>
  </note>

  <para>
   You should remember that DTrace scripts need to be carefully written and
   debugged, otherwise the trace information collected might
   be meaningless. In most cases where problems are found it is the
   instrumentation that is at fault, not the underlying system. When
   discussing information found using dynamic tracing, be sure to enclose
   the script used to allow that too to be checked and discussed.
  </para>
  </sect2>

  <sect2 id="defining-trace-points">
   <title>Defining New Probes</title>

  <para>
   New probes can be defined within the code wherever the developer
   desires, though this will require a recompilation. Below are the steps
   for inserting new probes:
  </para>

  <procedure>
   <step>
    <para>
     Decide on probe names and data to be made available through the probes
    </para>
   </step>

   <step>
    <para>
     Add the probe definitions to <filename>src/backend/utils/probes.d</filename>
    </para>
   </step>

   <step>
    <para>
     Include <filename>pg_trace.h</filename> if it is not already present in the
     module(s) containing the probe points, and insert
     <literal>TRACE_POSTGRESQL</literal> probe macros at the desired locations
     in the source code
    </para>
   </step>

   <step>
    <para>
     Recompile and verify that the new probes are available
    </para>
   </step>
  </procedure>

  <formalpara>
   <title>Example:</title>
   <para>
    Here is an example of how you would add a probe to trace all new
    transactions by transaction ID.
   </para>
  </formalpara>

  <procedure>
   <step>
    <para>
     Decide that the probe will be named <literal>transaction-start</literal> and
     requires a parameter of type <type>LocalTransactionId</type>
    </para>
   </step>

   <step>
    <para>
     Add the probe definition to <filename>src/backend/utils/probes.d</filename>:
<programlisting>
probe transaction__start(LocalTransactionId);
</programlisting>
     Note the use of the double underline in the probe name. In a DTrace
     script using the probe, the double underline needs to be replaced with a
     hyphen, so <literal>transaction-start</literal> is the name to document for
     users.
    </para>
   </step>

   <step>
    <para>
     At compile time, <literal>transaction__start</literal> is converted to a macro
     called <literal>TRACE_POSTGRESQL_TRANSACTION_START</literal> (notice the
     underscores are single here), which is available by including
     <filename>pg_trace.h</filename>.  Add the macro call to the appropriate location
     in the source code.  In this case, it looks like the following:

<programlisting>
TRACE_POSTGRESQL_TRANSACTION_START(vxid.localTransactionId);
</programlisting>
    </para>
   </step>

   <step>
    <para>
     After recompiling and running the new binary, check that your newly added
     probe is available by executing the following DTrace command.  You
     should see similar output:
<screen>
# dtrace -ln transaction-start
   ID    PROVIDER          MODULE           FUNCTION NAME
18705 postgresql49878     postgres     StartTransactionCommand transaction-start
18755 postgresql49877     postgres     StartTransactionCommand transaction-start
18805 postgresql49876     postgres     StartTransactionCommand transaction-start
18855 postgresql49875     postgres     StartTransactionCommand transaction-start
18986 postgresql49873     postgres     StartTransactionCommand transaction-start
</screen>
    </para>
   </step>
  </procedure>

  <para>
   There are a few things to be careful about when adding trace macros
   to the C code:

   <itemizedlist>
    <listitem>
     <para>
      You should take care that the data types specified for a probe's
      parameters match the data types of the variables used in the macro.
      Otherwise, you will get compilation errors.
     </para>
    </listitem>


    <listitem>
     <para>
      On most platforms, if <productname>PostgreSQL</productname> is
      built with <option>--enable-dtrace</option>, the arguments to a trace
      macro will be evaluated whenever control passes through the
      macro, <emphasis>even if no tracing is being done</emphasis>.  This is
      usually not worth worrying about if you are just reporting the
      values of a few local variables.  But beware of putting expensive
      function calls into the arguments.  If you need to do that,
      consider protecting the macro with a check to see if the trace
      is actually enabled:

<programlisting>
if (TRACE_POSTGRESQL_TRANSACTION_START_ENABLED())
    TRACE_POSTGRESQL_TRANSACTION_START(some_function(...));
</programlisting>

      Each trace macro has a corresponding <literal>ENABLED</literal> macro.
     </para>
    </listitem>
   </itemizedlist>

  </para>

  </sect2>

 </sect1>

</chapter>
