monitoring.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

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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</>, <command>top</>, <command>iostat</>, and <command>vmstat</>.
   Also, once one has identified a
   poorly-performing query, further investigation may be needed using
   <productname>PostgreSQL</productname>'s <xref linkend="sql-explain"
   endterm="sql-explain-title"> 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 <xref linkend="guc-stats-start-collector"> 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 <xref linkend="guc-stats-command-string">,
   <xref linkend="guc-stats-block-level">, and <xref
   linkend="guc-stats-row-level"> 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 <xref
   linkend="sql-set" endterm="sql-set-title"> 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, listed in <xref
   linkend="monitoring-stats-views-table">, 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 block and row access 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 unless altered while building the server).  So the
   displayed information lags behind actual activity.  Current-query
   information is reported to the collector immediately, but is still subject
   to the <varname>PGSTAT_STAT_INTERVAL</varname> delay before it becomes
   visible.
  </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 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 database OID, database name,
      process <acronym>ID</>, user OID, user name, current query, time at
      which the current query began execution, time at which the process
      was started, and client's address and port number.  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, the 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 database OID, database name,
      number of active server processes connected to that database,
      number of transactions committed and rolled back in that database,
      total disk blocks read, and total 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 (including TOAST tables),
      the table OID, schema and table name, number of sequential
      scans initiated, number of live rows fetched by sequential
      scans, number of index scans initiated (over all indexes
      belonging to the table), number of live rows fetched by index
      scans,
      and numbers 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 table and index OID, schema, table and index name,
      number of index scans initiated on that index, number of
      index entries returned by index scans, and number of live table rows
      fetched by simple index scans using that index.
      </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 (including TOAST tables),
      the table OID, schema and table name, number of disk
      blocks read from that table, number of buffer hits, numbers of
      disk blocks read and buffer hits in all indexes of that table,
      numbers of disk blocks read and buffer hits from that table's
      auxiliary TOAST table (if any), and 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 table and index OID, schema, table and index name,
      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 sequence OID, schema and sequence name,
      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>
   Beginning in <productname>PostgreSQL</productname> 8.1, indexes can be
   used either directly or via <quote>bitmap scans</>.  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