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

 <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 <firstterm>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.
   The <citetitle>User's Guide</citetitle> 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
   <firstterm>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</> (ie, waiting for a client command),
  <literal>idle in transaction</> (waiting for client inside a BEGIN block),
  or a command type name such as <literal>SELECT</>.  Also,
  <literal>waiting</> is attached if the server 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>
 </sect1>

 <sect1 id="monitoring-stats">
  <title>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 query 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 variables that are normally set in
   <filename>postgresql.conf</> (see <xref linkend="runtime-config"> for
   details about setting configuration variables).
  </para>

  <para>
   The variable <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 variables <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 runtime
   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 variables 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 server
   processes using the <command>SET</> command.  (To prevent ordinary users
   from hiding their activity from the administrator, only superusers are
   allowed to change these variables with <command>SET</>.)
  </para>

   <important>
    <para>
     Since the variables <varname>STATS_COMMAND_STRING</varname>,
     <varname>STATS_BLOCK_LEVEL</varname>,
     and <varname>STATS_ROW_LEVEL</varname>
     default to <literal>false</>, no statistics are actually collected
     in the default configuration!  You must turn one or more of them on
     before you will get useful results from the statistical display
     functions.
    </para>
   </important>

 </sect2>

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

  <para>
   Several predefined views are available to show the results of
   statistics collection.  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 PGSTAT_STAT_INTERVAL (500 milliseconds
   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.  It 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>
   <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 PID, database,
      user, and current query.  The current query column is only available
      to superusers; for others it reads as NULL.  (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 number of active backends,
      total transactions committed and total rolled back in that database,
      total disk blocks read, and total number of buffer hits (ie, 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 deletes.</entry>
     </row>

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

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

     <row>
      <entry><structname>pg_statio_user_tables</></entry>
      <entry>Same as 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 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 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 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 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.
  </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.  The per-database access functions accept a database OID to
   identify which database to report on.  The per-table and per-index
   functions accept 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 accept a backend ID number, which ranges from one to the number
   of currently active backends.
  </para>

  <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><function>pg_stat_get_db_numbackends</function>(<type>oid</type>)</entry>
      <entry><type>integer</type></entry>
      <entry>
       Number of active backends in database
      </entry>
     </row>

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

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

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

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

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

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

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

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

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

     <row>
      <entry><function>pg_stat_get_backend_idset</function>()</entry>
      <entry><type>set of integer</type></entry>
      <entry>
       Set of currently active backend IDs (from 1 to N where N is the
       number of active backends).  See usage example below.
      </entry>
     </row>

     <row>
      <entry><function>pg_stat_get_backend_pid</function>(<type>integer</type>)</entry>
      <entry><type>integer</type></entry>
      <entry>
       PID of backend process
      </entry>
     </row>

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

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

     <row>
      <entry><function>pg_stat_get_backend_activity</function>(<type>integer</type>)</entry>
      <entry><type>text</type></entry>
      <entry>
       Current query of backend process (NULL if caller is not superuser)
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
  Note: blocks_fetched minus blocks_hit gives the number of kernel 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>

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

<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>
</chapter>

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