monitoring.sgml

PostgreSQL7.4.6 for Linux

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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 rows returned by
      each type of scan, and totals of row 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 rows
      read, and the number of successfully fetched heap rows. (This may
      be less when there are index entries pointing to expired heap rows.)
      </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>