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<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</> and <command>top</>.  Also, once one has identified a
   poorly-performing query, further investigation may be needed using
   <productname>PostgreSQL</productname>'s <command>EXPLAIN</> command.
   <xref linkend="using-explain"> discusses <command>EXPLAIN</>
   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 platforms, <productname>PostgreSQL</productname> modifies its
   command title as reported by <command>ps</>, so that individual server
   processes can readily be identified.  A sample display is

<screen>
$ ps auxww | grep ^postgres
postgres   960  0.0  1.1  6104 1480 pts/1    SN   13:17   0:00 postmaster -i
postgres   963  0.0  1.1  7084 1472 pts/1    SN   13:17   0:00 postgres: stats buffer process   
postgres   965  0.0  1.1  6152 1512 pts/1    SN   13:17   0:00 postgres: stats collector process   
postgres   998  0.0  2.3  6532 2992 pts/1    SN   13:18   0:00 postgres: tgl runbug 127.0.0.1 idle
postgres  1003  0.0  2.4  6532 3128 pts/1    SN   13:19   0:00 postgres: tgl regression [local] SELECT waiting
postgres  1016  0.1  2.4  6532 3080 pts/1    SN   13:19   0:00 postgres: tgl regression [local] idle in transaction
</screen>

   (The appropriate invocation of <command>ps</> 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
   <application>postmaster</>, the master server process.  The command arguments
   shown for it are the same ones given when it was launched.  The next two
   processes implement the statistics collector, which will be described in
   detail in the next section.  (These will not be present if you have set
   the system not to start the statistics collector.)  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>database</> <replaceable>host</> <replaceable>activity</>
</screen>

  The user, database, and connection source host items remain the same for
  the life of the client connection, but the activity indicator changes.
  The activity may be <literal>idle</> (i.e., waiting for a client command),
  <literal>idle in transaction</> (waiting for client inside a <command>BEGIN</> block),
  or a command type name such as <literal>SELECT</>.  Also,
  <literal>waiting</> is attached if the server process is presently waiting
  on a lock held by another server process.  In the above example we can infer
  that process 1003 is waiting for process 1016 to complete its transaction and
  thereby release some lock or other.
  </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>postmaster</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</>
  output for each server process will be the original <command>postmaster</>
  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</>
   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 supports
   determining the exact command currently being executed by other server
   processes.
  </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</>.  (See <xref linkend="runtime-config"> for
   details about setting configuration parameters.)
  </para>

  <para>
   The parameter <varname>stats_start_collector</varname> must be set to
   <literal>true</> for the statistics collector to
   be launched at all.  This is the default and recommended setting,
   but it may be turned off if you have no interest in statistics and
   want to squeeze out every last drop of overhead.  (The savings is
   likely to be small, however.)  Note that this option
   cannot be changed while the server is running.
  </para>

  <para>
   The parameters <varname>stats_command_string</varname>,
   <varname>stats_block_level</varname>,
   and <varname>stats_row_level</varname> control how much information is
   actually sent to the collector and thus determine how much run-time
   overhead occurs.  These respectively determine whether a server process
   sends its current command string, disk-block-level access statistics, and
   row-level access statistics to the collector.  Normally these parameters are
   set in <filename>postgresql.conf</> so that they apply to all server
   processes, but it is possible to turn them on or off in individual sessions
   using the <command>SET</> command.  (To prevent ordinary users
   from hiding their activity from the administrator, only superusers are
   allowed to change these parameters with <command>SET</>.)
  </para>

   <note>
    <para>
     Since the parameters <varname>stats_command_string</varname>,
     <varname>stats_block_level</varname>, and
     <varname>stats_row_level</varname> default to <literal>false</>,
     very few statistics are collected in the default
     configuration. Enabling one or more of these configuration
     variables will significantly enhance the amount of useful data
     produced by the statistics collector, at the expense of
     additional run-time overhead.
    </para>
   </note>

 </sect2>

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

  <para>
   Several predefined views are available to show the results of
   statistics collection, listed in <xref
   linkend="monitoring-stats-views-table">.  Alternatively, one can
   build custom views using the underlying statistics functions.
  </para>

  <para>
   When using the statistics to monitor current activity, it is important
   to realize that the information does not update instantaneously.
   Each individual server process transmits new access counts to the collector
   just before waiting for another client command; so a query still in
   progress does not affect the displayed totals.  Also, the collector itself
   emits new totals at most once per <varname>pgstat_stat_interval</varname> milliseconds
   (500 by default).  So the displayed totals lag behind actual activity.
  </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 totals 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 appear not to change as long as you continue the
   current 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.
  </para>

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

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

    <tbody>
     <row>
      <entry><structname>pg_stat_activity</></entry>
      <entry>One row per server process, showing process
      <acronym>ID</>, database, user, current query, and the time at
      which the current query began execution. The columns that report
      data on the current query are only available if the parameter
      <varname>stats_command_string</varname> has been turned on.
      Furthermore, these columns read as null unless the user examining
      the view is a superuser or the same as the user owning the process
      being reported on.  (Note that because of the 
      collector's reporting delay, current query will only be up-to-date for 
      long-running queries.)</entry>
     </row>

     <row>
      <entry><structname>pg_stat_database</></entry>
      <entry>One row per database, showing the number of active backend server processes,
      total transactions committed and total rolled back in that database,
      total disk blocks read, and total number of buffer hits (i.e., block
      read requests avoided by finding the block already in buffer cache).
     </entry>
     </row>

     <row>
      <entry><structname>pg_stat_all_tables</></entry>
      <entry>For each table in the current database, total numbers of
      sequential and index scans, total numbers of tuples returned by
      each type of scan, and totals of tuple insertions, updates,
      and deletions.</entry>
     </row>

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

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

     <row>
      <entry><structname>pg_stat_all_indexes</></entry>
      <entry>For each index in the current database, the total number
      of index scans that have used that index, the number of index tuples
      read, and the number of successfully fetched heap tuples. (This may
      be less when there are index entries pointing to expired heap tuples.)
      </entry>
     </row>

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

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

     <row>
      <entry><structname>pg_statio_all_tables</></entry>
      <entry>For each table in the current database, the total number of disk
      blocks read from that table, the number of buffer hits, the numbers of
      disk blocks read and buffer hits in all the indexes of that table,
      the numbers of disk blocks read and buffer hits from the table's
      auxiliary TOAST table (if any), and the numbers of disk blocks read
      and buffer hits for the TOAST table's index.
      </entry>
     </row>

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

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

     <row>
      <entry><structname>pg_statio_all_indexes</></entry>
      <entry>For each index in the current database, the numbers of
      disk blocks read and buffer hits in that index.
      </entry>
     </row>

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

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

     <row>
      <entry><structname>pg_statio_all_sequences</></entry>
      <entry>For each sequence object in the current database, the numbers
      of disk blocks read and buffer hits in that sequence.
      </entry>
     </row>

     <row>
      <entry><structname>pg_statio_sys_sequences</></entry>
      <entry>Same as <structname>pg_statio_all_sequences</>, 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</></entry>
      <entry>Same as <structname>pg_statio_all_sequences</>, except that only user
      sequences are shown.</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_</> 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</>
   handles disk I/O, data that is not in the
   <productname>PostgreSQL</> buffer cache may still reside in the
   kernel's I/O cache, and may therefore still be fetched without
   requiring a physical read. Users interested in obtaining more
   detailed information on <productname>PostgreSQL</> I/O behavior are
   advised to use the <productname>PostgreSQL</> statistics collector
   in combination with operating system utilities that allow insight
   into the kernel's handling of I/O.
  </para>

  <para>
   Other ways of looking at the statistics can be set up by writing
   queries that use the same underlying statistics access functions as
   these standard views do.  These functions are listed in <xref
   linkend="monitoring-stats-funcs-table">.  The per-database access
   functions take a database OID as argument to identify which database to
   report on.  The per-table and per-index functions take a table or
   index OID.  (Note that only tables and indexes in the current
   database can be seen with these functions.)  The per-backend access
   functions take a backend ID number, which ranges from one to the
   number of currently active backend processes.
  </para>

  <table id="monitoring-stats-funcs-table">
   <title>Statistics Access 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_db_numbackends</function>(<type>oid</type>)</literal></entry>
      <entry><type>integer</type></entry>
      <entry>
       Number of active backend processes for database
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_db_xact_commit</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Transactions committed in database
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_db_xact_rollback</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Transactions rolled back in database
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_db_blocks_fetched</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Number of disk block fetch requests for database
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_db_blocks_hit</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Number of disk block fetch requests found in cache for database
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_numscans</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Number of sequential scans done when argument is a table,
       or number of index scans done when argument is an index
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_tuples_returned</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Number of tuples read by sequential scans when argument is a table,
       or number of index tuples read when argument is an index
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_tuples_fetched</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Number of valid (unexpired) table tuples fetched by sequential scans
       when argument is a table, or fetched by index scans using this index
       when argument is an index
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_tuples_inserted</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Number of tuples inserted into table
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_tuples_updated</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Number of tuples updated in table
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_tuples_deleted</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Number of tuples deleted from table
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_blocks_fetched</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Number of disk block fetch requests for table or index
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_blocks_hit</function>(<type>oid</type>)</literal></entry>
      <entry><type>bigint</type></entry>
      <entry>
       Number of disk block requests found in cache for table or index
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_idset</function>()</literal></entry>
      <entry><type>set of integer</type></entry>
      <entry>
       Set of currently active backend process IDs (from 1 to the
       number of active backend processes).  See usage example in the text.
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_backend_pid</function>()</literal></entry>
      <entry><type>integer</type></entry>
      <entry>
       Process ID of the backend process attached to the current session
      </entry>
     </row>

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

     <row>
      <entry><literal><function>pg_stat_get_backend_dbid</function>(<type>integer</type>)</literal></entry>
      <entry><type>oid</type></entry>
      <entry>
       Database ID of the given backend process
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_userid</function>(<type>integer</type>)</literal></entry>
      <entry><type>oid</type></entry>
      <entry>
       User ID of the given backend process
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_activity</function>(<type>integer</type>)</literal></entry>
      <entry><type>text</type></entry>
      <entry>
       Active command of the given backend process (null if the
       current user is not a superuser nor the same user as that of
       the session being queried, or
       <varname>stats_command_string</varname> is not on)
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_backend_activity_start</function>(<type>integer</type>)</literal></entry>
      <entry><type>timestamp with time zone</type></entry>
      <entry>
       The time at which the given backend process' currently
       executing query was started (null if the
       current user is not a superuser nor the same user as that of
       the session being queried, or
       <varname>stats_command_string</varname> is not on)
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_reset</function>()</literal></entry>
      <entry><type>boolean</type></entry>
      <entry>
       Reset all currently collected statistics
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

   <note>
    <para>
     <function>pg_stat_get_db_blocks_fetched</function> minus
     <function>pg_stat_get_db_blocks_hit</function> gives the number of kernel
     <function>read()</> calls issued for the table, index, or
     database; but the actual number of physical reads is usually
     lower due to kernel-level buffering.
    </para>
   </note>

  <para>
   The function <function>pg_stat_get_backend_idset</function> provides
   a convenient way to generate one row for each active backend process.  For
   example, to show the <acronym>PID</>s and current queries of all backend processes:

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

 </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
   <literal>pg_locks</literal> 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>

   For more information on locking and managing concurrency with
   <productname>PostgreSQL</productname>, refer to <xref linkend="mvcc">.
  </para>

  <note>
   <para>
    When the <literal>pg_locks</literal> view is accessed, the
    internal lock manager data structures are momentarily locked, and
    a copy is made for the view to display.  This ensures that the
    view produces a consistent set of results, while not blocking
    normal lock manager operations longer than necessary.  Nonetheless
    there could be some impact on database performance if this view is
    read often.
   </para>
  </note>

  <para>
   <xref linkend="monitoring-locks-table"> shows the definition of the
   <literal>pg_locks</literal> columns.  The
   <literal>pg_locks</literal> view contains one row per lockable
   object and requested lock mode.  Thus, the same lockable object may
   appear many times, if multiple transactions are holding or waiting
   for locks on it.  A lockable object is either a relation (e.g., a table) or a
   transaction ID.  (Note that this view includes only table-level
   locks, not row-level ones.  If a transaction is waiting for a
   row-level lock, it will appear in the view as waiting for the
   transaction ID of the current holder of that row lock.)
  </para>

  <table id="monitoring-locks-table">
   <title><literal>pg_locks</literal> Columns</title>

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

    <tbody>
     <row>
      <entry><structfield>relation</structfield></entry>
      <entry><type>oid</type></entry>
      <entry>
       The OID of the locked relation, or null if the lockable object
       is a transaction ID. This column can be joined with the column <literal>oid</> of the
       <literal>pg_class</literal> system catalog to get more
       information on the locked relation.  Note however that this
       will only work for relations in the current database (those for
       which the <structfield>database</structfield> column is either
       the current database's OID or zero).
      </entry>
     </row>

     <row>
      <entry><structfield>database</structfield></entry>
      <entry><type>oid</type></entry>
      <entry>
       The OID of the database in which the locked relation exists, or
       null if the lockable object is a transaction ID.  If the lock
       is on a globally-shared table, this field will be zero. This
       column can be joined with the column <literal>oid</> of the <literal>pg_database</literal>
       system catalog to get more information on the locked object's
       database.
      </entry>
     </row>

     <row>
      <entry><structfield>transaction</structfield></entry>
      <entry><type>xid</type></entry>
      <entry>
       The ID of a transaction, or null if the lockable object is a
       relation.  Every transaction holds an exclusive lock on its
       transaction ID for its entire duration.  If one transaction
       finds it necessary to wait specifically for another
       transaction, it does so by attempting to acquire share lock on
       the other transaction ID.  That will succeed only when the
       other transaction terminates and releases its locks.
      </entry>
     </row>

     <row>
      <entry><structfield>pid</structfield></entry>
      <entry><type>integer</type></entry>
      <entry>
       The process ID of the <productname>PostgreSQL</productname>
       server process belonging to the session that has acquired or is
       attempting to acquire the lock. If you have enabled the
       statistics collector, this column can be joined with the column
       <literal>pg_stat_activity</literal> view to get more
       information on the session holding or waiting to hold the
       lock.
      </entry>
     </row>

     <row>
      <entry><structfield>mode</structfield></entry>
      <entry><type>text</type></entry>
      <entry>
       The mode of the requested or held lock on the lockable
       object. For more information on the different lock modes
       available in <productname>PostgreSQL</productname>, refer to
       <xref linkend="mvcc">.
      </entry>
     </row>

     <row>
      <entry><structfield>isgranted</structfield></entry>
      <entry><type>boolean</type></entry>
      <entry>
       True if this lock has been granted (is held by this session).
       False indicates that this session is currently waiting to
       acquire this lock, which implies that some other session is
       holding a conflicting lock mode on the same lockable object.
       The waiting session will sleep until the other lock is released (or a
       deadlock situation is detected). A single session can be
       waiting to acquire at most one lock at a time.
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>
 </sect1>
</chapter>

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