postgresql/doc/src/sgml/monitoring.sgml

<!-- 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>top</>, <command>iostat</>, and <command>vmstat</>.
   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</>
   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</>, 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 postgres -i
postgres   963  0.0  1.1  7084 1472 pts/1    SN   13:17   0:00 postgres: writer 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
   master server process.  The command arguments
   shown for it are the same ones used when it was launched.  The next two
   processes are background worker processes automatically launched by the
   master process.  (The <quote>stats collector</> process 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 (client) host items remain the same for
  the life of the client connection, but the activity indicator changes.
  The activity can 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 appended if the server process is presently waiting
  on a lock held by another session.  In the above example we can infer
  that process 1003 is waiting for process 1016 to complete its transaction and
  thereby release some lock.
  </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</>
  output for each server process will be the original <command>postgres</>
  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 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 of the exact
   command currently being executed by other server processes.  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</>.  (See <xref linkend="runtime-config"> for
   details about setting configuration parameters.)
  </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-activities"> enables monitoring
   of the current command being executed by any server process.
  </para>

  <para>
   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"> command. (To prevent
   ordinary users from hiding their activity from the administrator,
   only superusers are allowed to change these parameters with
   <command>SET</>.)
  </para>

  <para>
   The statistics collector transmits the collected
   information to backends (including autovacuum) through temporary files.
   These files are stored in the <filename>pg_stat_tmp</filename> subdirectory.
   When the postmaster shuts down, a permanent copy of the statistics
   data is stored in the <filename>global</filename> subdirectory. For increased
   performance, the parameter <xref linkend="guc-stats-temp-directory"> can
   be pointed at a RAM-based file system, decreasing physical I/O requirements.
  </para>

 </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 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 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>pg_stat_xact_sys_tables</>,
   <structname>pg_stat_xact_user_tables</>, and
   <structname>pg_stat_xact_user_functions</>, or via these views' underlying
   functions (named the same as the standard statistics functions but with the
   prefix <function>pg_stat_get_xact_</function>).  These numbers do not act
   as stated above; instead they update continuously throughout the transaction.
  </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</structname>
       <indexterm><primary>pg_stat_activity</primary></indexterm>
      </entry>
      <entry>
       <para>One row per server process, showing information related to
        each connection to the server. Unless the
        <xref linkend="guc-track-activities"> parameter has been turned
         off, it is possible to monitor state and query information of
         all running processes.
       </para>
       <para>
        See <xref linkend="pg-stat-activity-view"> for more details.
       </para>
     </entry>
     </row>

     <row>
      <entry><structname>pg_stat_bgwriter</><indexterm><primary>pg_stat_bgwriter</primary></indexterm></entry>
      <entry>One row only, showing cluster-wide statistics. See
       <xref linkend="pg-stat-bgwriter-view"> for more details.
     </entry>
     </row>

     <row>
      <entry><structname>pg_stat_database</><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 more details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_database_conflicts</><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.
       Will only contain information on standby servers, since
       conflicts do not occur on master servers.
       See <xref linkend="pg-stat-database-conflicts-view"> for more details.
     </entry>
     </row>

     <row>
      <entry><structname>pg_stat_replication</><indexterm><primary>pg_stat_replication</primary></indexterm></entry>
      <entry>One row per WAL sender process, showing statistics about the
       replication to this slave. See <xref linkend="pg-stat-replication-view">
       for more details. Only directly connected standbys are listed; no
       information about downstream standby servers is recorded.
     </entry>
     </row>

     <row>
      <entry><structname>pg_stat_all_tables</><indexterm><primary>pg_stat_all_tables</primary></indexterm></entry>
      <entry>
       One row for each table in the current database (including TOAST
       tables) with information about accesses to this specific table.
       See <xref linkend="pg-stat-all-tables-view"> for more details.
      </entry>
     </row>

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

     <row>
      <entry><structname>pg_stat_xact_all_tables</><indexterm><primary>pg_stat_xact_all_tables</primary></indexterm></entry>
      <entry>Similar to <structname>pg_stat_all_tables</>, but counts actions
      taken so far within the current transaction (which are <emphasis>not</>
      yet included in <structname>pg_stat_all_tables</> 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</><indexterm><primary>pg_stat_xact_sys_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_xact_all_tables</>, except that only
      system tables are shown.</entry>
     </row>

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

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

     <row>
      <entry><structname>pg_stat_sys_indexes</><indexterm><primary>pg_stat_sys_indexes</primary></indexterm></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</><indexterm><primary>pg_stat_user_indexes</primary></indexterm></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</><indexterm><primary>pg_statio_all_tables</primary></indexterm></entry>
      <entry>
       One row for each table in the current database (including TOAST
       tables) with information about I/O on this specific table.
       See <xref linkend="pg-statio-all-tables-view"> for more details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_statio_sys_tables</><indexterm><primary>pg_statio_sys_tables</primary></indexterm></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</><indexterm><primary>pg_statio_user_tables</primary></indexterm></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</><indexterm><primary>pg_statio_all_indexes</primary></indexterm></entry>
      <entry>
       One row for each index in the current database
       with information about I/O on this specific index.
       See <xref linkend="pg-statio-all-indexes-view"> for more details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_statio_sys_indexes</><indexterm><primary>pg_statio_sys_indexes</primary></indexterm></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</><indexterm><primary>pg_statio_user_indexes</primary></indexterm></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</><indexterm><primary>pg_statio_all_sequences</primary></indexterm></entry>
     <entry>
       One row for each sequence in the current database
       with information about I/O on this specific sequence.
       See <xref linkend="pg-statio-all-sequences-view"> for more details.
     </entry>
     </row>

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

     <row>
      <entry><structname>pg_stat_user_functions</><indexterm><primary>pg_stat_user_functions</primary></indexterm></entry>
      <entry>
       One row for each tracked function (as specified by the
       <xref linkend="guc-track-functions"> parameter). See
       <xref linkend="pg-stat-user-functions-view"> for more details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_xact_user_functions</><indexterm><primary>pg_stat_xact_user_functions</primary></indexterm></entry>
      <entry>Similar to <structname>pg_stat_user_functions</>, but counts only
      calls during the current transaction (which are <emphasis>not</>
      yet included in <structname>pg_stat_user_functions</>).</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>
   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
   <structname>pg_stat_all_indexes</>.<structfield>idx_tup_read</>
   count(s) for the index(es) it uses, and it increments the
   <structname>pg_stat_all_tables</>.<structfield>idx_tup_fetch</>
   count for the table, but it does not affect
   <structname>pg_stat_all_indexes</>.<structfield>idx_tup_fetch</>.
  </para>

  <note>
   <para>
    Before <productname>PostgreSQL</productname> 8.1, the
    <structfield>idx_tup_read</> and <structfield>idx_tup_fetch</> counts
    were essentially always equal.  Now they can be different even without
    considering bitmap scans, because <structfield>idx_tup_read</> counts
    index entries retrieved from the index while <structfield>idx_tup_fetch</>
    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>

  <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 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</> 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 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 function-call statistics
   take a function OID.  (Note that only tables, indexes, and functions
   in the current database can be seen with these functions.)  The
   per-server-process access functions take a server process
   number, which ranges from one to the number of currently active
   server processes.
  </para>


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

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

   <tbody>
    <row>
     <entry>datid</entry>
     <entry><type>oid</></entry>
     <entry>The OID of the database the backend is connected to.
     This value can also be returned by directly calling
     the <function>pg_stat_get_backend_dbid</function> function.</entry>
    </row>
    <row>
     <entry>datname</entry>
     <entry><type>name</></entry>
     <entry>The name of the database the backend is connected to.</entry>
    </row>
    <row>
     <entry>pid</entry>
     <entry><type>integer</></entry>
     <entry>The process ID of the backend.
     This value can also be returned by directly calling
     the <function>pg_stat_get_backend_pid</function>.</entry>
    </row>
    <row>
     <entry>usesysid</entry>
     <entry><type>oid</></entry>
     <entry>The id of the user logged into the backend.
     This value can also be returned by directly calling
     the <function>pg_stat_get_backend_userid</function>.</entry>
    </row>
    <row>
     <entry>usename</entry>
     <entry><type>name</></entry>
     <entry>The name of the user logged into the backend.</entry>
    </row>
    <row>
     <entry>application_name</entry>
     <entry><type>text</></entry>
     <entry>The name of the application that has initiated the connection
      to the backend.</entry>
    </row>
    <row>
     <entry>client_addr</entry>
     <entry><type>inet</></entry>
     <entry>The remote IP of the client connected to the backend.
      If this field is not set, it indicates that the client is either connected
      via a Unix socket on the server machine or is an internal process such
      as autovacuum.
      This value can also be returned by directly calling
      the <function>pg_stat_get_backend_client_addr</function>.
     </entry>
    </row>
    <row>
     <entry>client_hostname</entry>
     <entry><type>text</></entry>
     <entry>
      If available, the hostname of the client as reported by a
      reverse lookup of <structfield>client_addr</>. This field will
      only be set when <xref linkend="guc-log-hostname"> is enabled.
     </entry>
    </row>
    <row>
     <entry>client_port</entry>
     <entry><type>integer</></entry>
     <entry>
      The remote TCP port that the client is using for communication
      to the backend, or <symbol>NULL</> if a unix socket is used.
      This value can also be returned by directly calling
      the <function>pg_stat_get_backend_client_port</function>.
     </entry>
    </row>
    <row>
     <entry>backend_start</entry>
     <entry><type>timestamp with time zone</></entry>
     <entry>
      The time when this process was started,  i.e. when the
      client connected to the server.
      This value can also be returned by directly calling
      the <function>pg_stat_get_backend_start</function>.
     </entry>
    </row>
    <row>
     <entry>xact_start</entry>
     <entry><type>timestamp with time zone</></entry>
     <entry>
      The time when the current transaction was started. If the client is
      using autocommit for transactions, this value is equal to the
      query_start column.
      This value can also be returned by directly calling
      the <function>pg_stat_get_backend_xact_start</function>.
     </entry>
    </row>
    <row>
     <entry>query_start</entry>
     <entry><type>timestamp with time zone</></entry>
     <entry>
      The time when the currently active query started, or if
      <structfield>state</> is <literal>idle</>, when the last query
      was started.
      This value can also be returned by directly calling
      the <function>pg_stat_get_backend_activity_start</function>.
     </entry>
    </row>
    <row>
     <entry>state_change</entry>
     <entry><type>timestamp with time zone</></entry>
     <entry>The time when the <structfield>state</> was last changed.</entry>
    </row>
    <row>
     <entry>waiting</entry>
     <entry><type>boolean</></entry>
     <entry>
      Boolean indicating if a backend is currently waiting on a lock.
      This value can also be returned by directly calling
      the <function>pg_stat_get_backend_waiting</function>.
     </entry>
    </row>
    <row>
     <entry>state</entry>
     <entry><type>text</></entry>
     <entry>
       The <structfield>state</> of the currently running query.
       Can be one of:
       <variablelist>
         <varlistentry>
           <term>active</term>
           <listitem>
             <para>
              The backend is executing a query.
             </para>
           </listitem>
           </varlistentry>
           <varlistentry>
             <term>idle</term>
           <listitem>
             <para>
               There is no query executing in the backend.
             </para>
           </listitem>
         </varlistentry>
         <varlistentry>
           <term>idle in transaction</term>
           <listitem>
             <para>
               The backend is in a transaction, but is currently not currently
               executing a query.
             </para>
           </listitem>
         </varlistentry>
         <varlistentry>
           <term>idle in transaction (aborted)</term>
           <listitem>
             <para>
               This state is similar to <literal>idle in transaction</>,
               except one of the statements in the transaction caused an error.
             </para>
           </listitem>
         </varlistentry>
         <varlistentry>
           <term>fastpath function call</term>
           <listitem>
             <para>
               The backend is executing a fast-path function.
             </para>
           </listitem>
         </varlistentry>
         <varlistentry>
           <term>disabled</term>
           <listitem>
             <para>
               This state indicates that <xref linkend="guc-track-activities">
               is disabled.
             </para>
           </listitem>
         </varlistentry>
       </variablelist>
      <note>
       <para>
        The <structfield>waiting</> and <structfield>state</> columns are
        independent.  If a query is in the <literal>active</> state,
        it may or may not be <literal>waiting</>.  If a query is
        <literal>active</> and <structfield>waiting</> is true, it means
        that the query is being executed, but is being blocked by a lock
        somewhere in the system.
       </para>
      </note>
     </entry>
    </row>
    <row>
     <entry>query</entry>
     <entry><type>text</></entry>
     <entry>
      The most recent query that the backend has executed. If
      <structfield>state</> is <literal>active</> this means the currently
      executing query. In all other states, it means the last query that was
      executed.
     </entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_activity</structname> view will have one row
   per server process, showing information related to each connection to
   the server.
  </para>
  <para>
   All functions used in the view are indexed by backend id number. The
   function <function>pg_stat_get_backend_idset</function> provides a
   convenient way to generate one row for each active server process. For
   example, to show the <acronym>PID</>s and current queries of all server processes:

<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="pg-stat-bgwriter-view" xreflabel="pg_stat_bgwriter">
   <title>pg_stat_bgwriter view</title>

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

    <tbody>
     <row>
      <entry>checkpoints_timed</entry>
      <entry><type>bigint</type></entry>
      <entry>Number of scheduled checkpoints.
     This value can also be returned by directly calling
     the <function>pg_stat_get_bgwriter_timed_checkpoints</function> function.</entry>
     </row>
     <row>
      <entry>checkpoints_requested</entry>
      <entry><type>bigint</type></entry>
      <entry>Number of requested checkpoints.
     This value can also be returned by directly calling
     the <function>pg_stat_get_bgwriter_requested_checkpoints</function> function.</entry>
     </row>
     <row>
      <entry>buffers_checkpoint</entry>
      <entry><type>bigint</type></entry>
      <entry>Number of buffers written during checkpoints.
     This value can also be returned by directly calling
     the <function>pg_stat_get_bgwriter_buf_written_checkpoints</function> function.</entry>
     </row>
     <row>
      <entry>buffers_clean</entry>
      <entry><type>bigint</type></entry>
      <entry>Number of buffers written by the background writer.
     This value can also be returned by directly calling
     the <function>pg_stat_get_bgwriter_buf_written_clean</function> function.</entry>
     </row>
     <row>
      <entry>maxwritten_clean</entry>
      <entry><type>bigint</type></entry>
      <entry>Number of times the background writer stopped a cleaning
      scan because it had written too many buffers.
     This value can also be returned by directly calling
     the <function>pg_stat_get_bgwriter_maxwritten_clean</function> function.</entry>
     </row>
     <row>
      <entry>buffers_backend</entry>
      <entry><type>bigint</type></entry>
      <entry>Number of buffers written directly by a backend.
     This value can also be returned by directly calling
     the <function>pg_stat_get_buf_written_backend</function> function.</entry>
     </row>
     <row>
      <entry>buffers_backend_fsync</entry>
      <entry><type>bigint</type></entry>
      <entry>Number of times a backend had to execute its own fsync
      call (normally the background writer handles those even when the
      backend does its own write)</entry>
     </row>
     <row>
      <entry>buffers_alloc</entry>
      <entry><type>bigint</type></entry>
      <entry>Number of buffers allocated.
     This value can also be returned by directly calling
     the <function>pg_stat_get_buf_alloc</function> function.</entry>
     </row>
     <row>
      <entry>stats_reset</entry>
      <entry><type>bigint</type></entry>
      <entry>The last time these statistics were reset.
     This value can also be returned by directly calling
     the <function>pg_stat_get_bgwriter_stat_reset_time</function> function.</entry>
     </row>
    </tbody>
    </tgroup>
  </table>

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

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

   <tbody>
    <row>
     <entry>datid</entry>
     <entry><type>oid</></entry>
     <entry>The OID of the database</entry>
    </row>
    <row>
     <entry>datname</entry>
     <entry><type>name</></entry>
     <entry>The name of the database</entry>
    </row>
    <row>
     <entry>numbackends</entry>
     <entry><type>integer</></entry>
     <entry>The number of backends currently connected to this database.
     This is the only column in this view that returns a value for the
     current state, all other columns return the accumulated values since
     the last reset. This value can also be returned by directly calling
     the <function>pg_stat_get_db_numbackends</function> function.</entry>
    </row>
    <row>
     <entry>xact_commit</entry>
     <entry><type>bigint</></entry>
     <entry>The number of transactions in this database that have been
     committed. This value can also be returned by directly calling
     the <function>pg_stat_get_db_xact_commit</function> function.</entry>
    </row>
    <row>
     <entry>xact_rollback</entry>
     <entry><type>bigint</></entry>
     <entry>The number of transactions in this database that have been
     rolled back. This value can also be returned by directly calling
     the <function>pg_stat_get_db_xact_rollback</function> function.</entry>
    </row>
    <row>
     <entry>blks_read</entry>
     <entry><type>bigint</></entry>
     <entry>The number of disk blocks read in this database.
     This value can also be returned by directly calling
     the <function>pg_stat_get_db_blocks_fetched</function> and
     <function>pg_stat_get_db_blocks_hit</function> functions and
     subtracting the results.</entry>
    </row>
    <row>
     <entry>blks_hits</entry>
     <entry><type>bigint</></entry>
     <entry>The number of disk blocks read from the buffer cache
     (this only includes hits in the PostgreSQL buffer cache, and not
     the operating system filesystem cache).
     This value can also be returned by directly calling
     the <function>pg_stat_get_db_blocks_hit</function> function.</entry>
    </row>
    <row>
     <entry>tup_returned</entry>
     <entry><type>bigint</></entry>
     <entry>The number of rows returned by queries in this database.
     This value can also be returned by directly calling
     the <function>pg_stat_get_db_tuples_returned</function> function.</entry>
    </row>
    <row>
     <entry>tup_fetched</entry>
     <entry><type>bigint</></entry>
     <entry>The number of rows fetched by queries in this database.
     This value can also be returned by directly calling
     the <function>pg_stat_get_db_tuples_fetched</function> function.</entry>
    </row>
    <row>
     <entry>tup_inserted</entry>
     <entry><type>bigint</></entry>
     <entry>The number of rows inserted by queries in this database.
     This value can also be returned by directly calling
     the <function>pg_stat_get_db_tuples_inserted</function> function.</entry>
    </row>
    <row>
     <entry>tup_updated</entry>
     <entry><type>bigint</></entry>
     <entry>The number of rows updated by queries in this database.
     This value can also be returned by directly calling
     the <function>pg_stat_get_db_tuples_updated</function> function.</entry>
    </row>
    <row>
     <entry>tup_deleted</entry>
     <entry><type>bigint</></entry>
     <entry>The number of rows deleted by queries in this database.
     This value can also be returned by directly calling
     the <function>pg_stat_get_db_tuples_deleted</function> function.</entry>
    </row>
    <row>
     <entry>conflicts</entry>
     <entry><type>bigint</></entry>
     <entry>
      The number of queries canceled due to conflict with recovery
      (on standby servers) in this database. (See
      <xref linkend="pg-stat-database-conflicts-view"> for more details).
      This value can also be returned by directly calling
      the <function>pg_stat_get_db_conflict_all</function> function.
     </entry>
    </row>
    <row>
     <entry>temp_files</entry>
     <entry><type>bigint</></entry>
     <entry>
      The number of temporary files written by queries in the database.
      All temporary files are counted, regardless of why the temporary file
      was created (sorting or hash) or file size, and regardless of the
      <xref linkend="guc-log-temp-files"> setting.
      This value can also be returned by directly calling
      the <function>pg_stat_get_db_temp_files</function> function.
     </entry>
    </row>
    <row>
     <entry>temp_bytes</entry>
     <entry><type>bigint</></entry>
     <entry>
      The amount of data written to temporary files by queries in
      the database. All temporary files are counted, regardless of why
      the temporary file was created (sorting or hash) or file size, and
      regardless of the <xref linkend="guc-log-temp-files"> setting.
      This value can also be returned by directly calling
      the <function>pg_stat_get_db_temp_bytes</function> function.
     </entry>
    </row>
    <row>
     <entry>deadlocks</entry>
     <entry><type>bigint</></entry>
     <entry>Number of deadlocks detected in the database.
      This value can also be returned by directly calling
      the <function>pg_stat_get_db_deadlocks</function> function.</entry>
    </row>
    <row>
     <entry>stats_reset</entry>
     <entry><type>timestamptz</></entry>
     <entry>The last time the statistics were reset.
      This value can also be returned by directly calling
      the <function>pg_stat_get_reset_time</function> function.</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>pg_stat_database_conflicts view</title>
   <tgroup cols="3">
    <thead>
    <row>
      <entry>Column</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry>datid</entry>
     <entry><type>oid</></entry>
     <entry>The OID of the database</entry>
    </row>
    <row>
     <entry>datname</entry>
     <entry><type>name</></entry>
     <entry>The name of the database</entry>
    </row>
    <row>
     <entry>confl_tablespace</entry>
     <entry><type>bigint</></entry>
     <entry>The number of queries that have been canceled due to
      dropped tablespaces. This value can also be returned by directly calling
      the <function>pg_stat_get_db_conflict_tablespace</function> function.</entry>
    </row>
    <row>
     <entry>confl_lock</entry>
     <entry><type>bigint</></entry>
     <entry>The number of queries that have been canceled due to
      lock timeouts. This value can also be returned by directly calling
      the <function>pg_stat_get_db_conflict_lock</function> function.</entry>
    </row>
    <row>
     <entry>confl_snapshot</entry>
     <entry><type>bigint</></entry>
     <entry>The number of queries that have been canceled due to
      old snapshots. This value can also be returned by directly calling
      the <function>pg_stat_get_db_conflict_snapshot</function> function.</entry>
    </row>
    <row>
     <entry>confl_bufferpin</entry>
     <entry><type>bigint</></entry>
     <entry>The number of queries that have been canceled due to
      pinned buffers. This value can also be returned by directly calling
      the <function>pg_stat_get_db_conflict_bufferpin</function> function.</entry>
    </row>
    <row>
     <entry>confl_deadlock</entry>
     <entry><type>bigint</></entry>
     <entry>The number of queries that have been canceled due to
      deadlocks. This value can also be returned by directly calling
      the <function>pg_stat_get_db_conflict_startup_deadlock</function>
      function.</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 due to conflict with recovery on standby servers.
   Will only contain information on standby servers, since
   conflicts do not occur on master servers.
  </para>

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

   <tbody>
    <row>
     <entry>pid</entry>
     <entry><type>integer</></entry>
     <entry>The process id of the WAL sender process</entry>
    </row>
    <row>
     <entry>usesysid</entry>
     <entry><type>oid</></entry>
     <entry>The OID of the user logged into this WAL sender process</entry>
    </row>
    <row>
     <entry>usename</entry>
     <entry><type>name</></entry>
     <entry>The name of the user logged into this WAL sender process</entry>
    </row>
    <row>
     <entry>application_name</entry>
     <entry><type>text</></entry>
     <entry>The name of the application that has initiated the connection
      to the WAL sender.</entry>
    </row>
    <row>
     <entry>client_addr</entry>
     <entry><type>inet</></entry>
     <entry>The remote IP of the client connected to the WAL sender.
      If this field is not set, it indicates that the client is
      connected via a Unix socket on the server machine.
     </entry>
    </row>
    <row>
     <entry>client_hostname</entry>
     <entry><type>text</></entry>
     <entry>
      If available, the hostname of the client as reported by a
      reverse lookup of <structfield>client_addr</>. This field will
      only be set when <xref linkend="guc-log-hostname"> is enabled.
     </entry>
    </row>
    <row>
     <entry>client_port</entry>
     <entry><type>integer</></entry>
     <entry>
      The remote TCP port that the client is using for communication
      to the, or <symbol>NULL</> if a unix socket is used.
     </entry>
    </row>
    <row>
     <entry>backend_start</entry>
     <entry><type>timestamp with time zone</></entry>
     <entry>
      The time when this process was started,  i.e. when the
      client connected to the WAL sender.
     </entry>
    </row>
    <row>
     <entry>state</entry>
     <entry><type>text</></entry>
     <entry>Current WAL sender state</entry>
    </row>
    <row>
     <entry>sent_location</entry>
     <entry><type>text</></entry>
     <entry>Last transaction log position sent on this connection</entry>
    </row>
    <row>
     <entry>write_location</entry>
     <entry><type>text</></entry>
     <entry>Last transaction log position written to disk by the slave</entry>
    </row>
    <row>
     <entry>flush_location</entry>
     <entry><type>text</></entry>
     <entry>Last transaction log position flushed to disk by the slave</entry>
    </row>
    <row>
     <entry>replay_location</entry>
     <entry><type>text</></entry>
     <entry>Last transaction log position replayed into the database on the slave</entry>
    </row>
    <row>
     <entry>sync_priority</entry>
     <entry><type>int</></entry>
     <entry>
      The priority in the order which this slave will be picked as
      the synchronous standby.
     </entry>
    </row>
    <row>
     <entry>sync_state</entry>
     <entry><type>text</></entry>
     <entry>
      The synchronous state of this slave.
     </entry>
    </row>
   </tbody>
   </tgroup>
  </table>

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

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

   <tbody>
    <row>
     <entry>relid</entry>
     <entry><type>oid</></entry>
     <entry>The OID of the table</entry>
    </row>
    <row>
     <entry>schemaname</entry>
     <entry><type>name</></entry>
     <entry>The name of the schema that the table is in</entry>
    </row>
    <row>
     <entry>relname</entry>
     <entry><type>name</></entry>
     <entry>The name of the table</entry>
    </row>
    <row>
     <entry>seq_scan</entry>
     <entry><type>bigint</></entry>
     <entry>The number of sequential scans initiated on this table.
     This value can also be returned by directly calling
     the <function>pg_stat_get_numscans</function> function.</entry>
    </row>
    <row>
     <entry>seq_tup_read</entry>
     <entry><type>bigint</></entry>
     <entry>The number of live rows fetch by sequential scans.
     This value can also be returned by directly calling
     the <function>pg_stat_get_tuples_returned</function> function.</entry>
    </row>
    <row>
     <entry>idx_scan</entry>
     <entry><type>bigint</></entry>
     <entry>The number of index scans initiated on this table</entry>
    </row>
    <row>
     <entry>idx_tup_fetch</entry>
     <entry><type>bigint</></entry>
     <entry>The number of live rows fetch by index scans.</entry>
    </row>
    <row>
     <entry>n_tup_ins</entry>
     <entry><type>bigint</></entry>
     <entry>The number of rows inserted.
     This value can also be returned by directly calling
     the <function>pg_stat_get_tuples_inserted</function> function.</entry>
    </row>
    <row>
     <entry>n_tup_upd</entry>
     <entry><type>bigint</></entry>
     <entry>The number of rows updated.
     This value can also be returned by directly calling
     the <function>pg_stat_get_tuples_updated</function> function.</entry>
    </row>
    <row>
     <entry>n_tup_del</entry>
     <entry><type>bigint</></entry>
     <entry>The number of rows deleted.
     This value can also be returned by directly calling
     the <function>pg_stat_get_tuples_deleted</function> function.</entry>
    </row>
    <row>
     <entry>n_tup_hot_upd</entry>
     <entry><type>bigint</></entry>
     <entry>The number of rows HOT (i.e., no separate index update) updated.
     This value can also be returned by directly calling
     the <function>pg_stat_get_tuples_hot_updated</function> function.</entry>
    </row>
    <row>
     <entry>n_live_tup</entry>
     <entry><type>bigint</></entry>
     <entry>The number of live rows.
     This value can also be returned by directly calling
     the <function>pg_stat_get_live_tuples</function> function.</entry>
    </row>
    <row>
     <entry>n_dead_tup</entry>
     <entry><type>bigint</></entry>
     <entry>The number of dead rows.
     This value can also be returned by directly calling
     the <function>pg_stat_get_dead_tuples</function> function.</entry>
    </row>
    <row>
     <entry>last_vacuum</entry>
     <entry><type>timestamp with time zone</></entry>
     <entry>The last time the table was manually non-<option>FULL</> vacuumed.
     This value can also be returned by directly calling
     the <function>pg_stat_get_last_vacuum_time</function> function.</entry>
    </row>
    <row>
     <entry>last_autovacuum</entry>
     <entry><type>timestamp with time zone</></entry>
     <entry>The last time the table was vacuumed by the autovacuum daemon.
     This value can also be returned by directly calling
     the <function>pg_stat_get_last_autovacuum_time</function> function.</entry>
    </row>
    <row>
     <entry>last_analyze</entry>
     <entry><type>timestamp with time zone</></entry>
     <entry>The last time the table was manually analyzed.
     This value can also be returned by directly calling
     the <function>pg_stat_get_last_analyze_time</function> function.</entry>
    </row>
    <row>
     <entry>last_autoanalyze</entry>
     <entry><type>timestamp with time zone</></entry>
     <entry>The last time the table was analyzed by the autovacuum daemon.
     This value can also be returned by directly calling
     the <function>pg_stat_get_last_autoanalyze_time</function> function.</entry>
    </row>
    <row>
     <entry>vacuum_count</entry>
     <entry><type>bigint</></entry>
     <entry>The number of times this table has been manually non-<option>FULL</> vacuumed.
     This value can also be returned by directly calling
     the <function>pg_stat_get_vacuum_count</function> function.</entry>
    </row>
    <row>
     <entry>autovacuum_count</entry>
     <entry><type>bigint</></entry>
     <entry>The number of times this table has been vacuumed by the autovacuum daemon.
     This value can also be returned by directly calling
     the <function>pg_stat_get_autovacuum_count</function> function.</entry>
    </row>
    <row>
     <entry>analyze_count</entry>
     <entry><type>bigint</></entry>
     <entry>The number of times this table has been manually analyzed.
     This value can also be returned by directly calling
     the <function>pg_stat_get_analyze_count</function> function.</entry>
    </row>
    <row>
     <entry>autoanalyze_count</entry>
     <entry><type>bigint</></entry>
     <entry>The number of times this table has been analyzed by the autovacuum daemon.
     This value can also be returned by directly calling
     the <function>pg_stat_get_autoanalyze_count</function> function.</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) with information about accesses to this specific table. The
   <structname>pg_stat_user_tables</structname> and
   <structname>pg_stat_sys_tables</structname> contain the same information,
   but filtered to only have rows for user and system tables.
  </para>

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

   <tbody>
    <row>
     <entry>relid</entry>
     <entry><type>oid</></entry>
     <entry>The OID of the table for this index</entry>
    </row>
    <row>
     <entry>indexrelid</entry>
     <entry><type>oid</></entry>
     <entry>The OID of the index</entry>
    </row>
    <row>
     <entry>schemaname</entry>
     <entry><type>name</></entry>
     <entry>The name of the schema the index is in</entry>
    </row>
    <row>
     <entry>relname</entry>
     <entry><type>name</></entry>
     <entry>The name of the table for this index</entry>
    </row>
    <row>
     <entry>indexrelname</entry>
     <entry><type>name</></entry>
     <entry>The name of the index</entry>
    </row>
    <row>
     <entry>idx_scan</entry>
     <entry><type>bigint</></entry>
     <entry>Number of index scans initiated on this index.
     This value can also be returned by directly calling
     the <function>pg_stat_get_numscans</function> function.</entry>
    </row>
    <row>
     <entry>idx_tup_read</entry>
     <entry><type>bigint</></entry>
     <entry>Number of index entries returned by scans on this index.
     This value can also be returned by directly calling
     the <function>pg_stat_get_tuples_returned</function> function.</entry>
    </row>
    <row>
     <entry>idx_tup_fetch</entry>
     <entry><type>bigint</></entry>
     <entry>Number of live table rows fetched by simple index scans using this index.
     This value can also be returned by directly calling
     the <function>pg_stat_get_tuples_fetched</function> function.</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
   with information about accesses to this specific index. The
   <structname>pg_stat_user_indexes</structname> and
   <structname>pg_stat_sys_indexes</structname> contain the same information,
   but filtered to only have rows for user and system indexes.
  </para>

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

   <tbody>
    <row>
     <entry>relid</entry>
     <entry><type>oid</></entry>
     <entry>The OID of the table</entry>
    </row>
    <row>
     <entry>schemaname</entry>
     <entry><type>name</></entry>
     <entry>The name of the schema that the table is in</entry>
    </row>
    <row>
     <entry>relname</entry>
     <entry><type>name</></entry>
     <entry>The name of the table</entry>
    </row>
    <row>
     <entry>heap_blks_read</entry>
     <entry><type>name</></entry>
     <entry>Number of disk blocks read from this table.
     This value can also be returned by directly calling
     the <function>pg_stat_get_blocks_fetched</function> and
     <function>pg_stat_get_blocks_hit</function> functions and
     subtracting the results.</entry>
    </row>
    <row>
     <entry>heap_blks_hit</entry>
     <entry><type>name</></entry>
     <entry>Number of buffer hits in this table.
     This value can also be returned by directly calling
     the <function>pg_stat_get_blocks_hit</function> function.</entry>
    </row>
    <row>
     <entry>idx_blks_read</entry>
     <entry><type>name</></entry>
     <entry>Number of disk blocks read from all indexes on this table</entry>
    </row>
    <row>
     <entry>idx_blks_hit</entry>
     <entry><type>name</></entry>
     <entry>Number of buffer hits in all indexes of this table.</entry>
    </row>
    <row>
     <entry>toast_blks_read</entry>
     <entry><type>name</></entry>
     <entry>Number of disk blocks read from this table's TOAST table (if any)</entry>
    </row>
    <row>
     <entry>toast_blks_hit</entry>
     <entry><type>name</></entry>
     <entry>Number of buffer hits in this table's TOAST table (if any)</entry>
    </row>
    <row>
     <entry>tidx_blks_read</entry>
     <entry><type>name</></entry>
     <entry>Number of disk blocks read from this table's TOAST table index (if any)</entry>
    </row>
    <row>
     <entry>tidx_blks_hit</entry>
     <entry><type>name</></entry>
     <entry>Number of buffer hits in this table's TOAST table index (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) with information about I/O on this specific table. The
   <structname>pg_statio_user_tables</structname> and
   <structname>pg_statio_sys_tables</structname> contain the same information,
   but filtered to only have rows for user and system tables.
  </para>

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

   <tbody>
    <row>
     <entry>relid</entry>
     <entry><type>oid</></entry>
     <entry>The OID of the table for this index</entry>
    </row>
    <row>
     <entry>indexrelid</entry>
     <entry><type>oid</></entry>
     <entry>The OID of the index</entry>
    </row>
    <row>
     <entry>schemaname</entry>
     <entry><type>name</></entry>
     <entry>The name of the schema the index is in</entry>
    </row>
    <row>
     <entry>relname</entry>
     <entry><type>name</></entry>
     <entry>The name of the table for this index</entry>
    </row>
    <row>
     <entry>indexrelname</entry>
     <entry><type>name</></entry>
     <entry>The name of the index</entry>
    </row>
    <row>
     <entry>idx_blks_read</entry>
     <entry><type>name</></entry>
     <entry>Number of disk blocks read from the index.
     This value can also be returned by directly calling
     the <function>pg_stat_get_blocks_fetched</function> and
     <function>pg_stat_get_blocks_hit</function> functions and
     subtracting the results.</entry>
    </row>
    <row>
     <entry>idx_blks_hit</entry>
     <entry><type>name</></entry>
     <entry>Number of buffer hits in the index.
     This value can also be returned by directly calling
     the <function>pg_stat_get_blocks_hit</function> function.</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
   with information about I/O on this specific index. The
   <structname>pg_statio_user_indexes</structname> and
   <structname>pg_statio_sys_indexes</structname> contain the same information,
   but filtered to only have rows for user and system indexes.
  </para>

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

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

  <para>
   The <structname>pg_statio_all_indexes</structname> view will contain
   one row for each sequence in the current database
   with information about I/O on this specific sequence.
  </para>

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

   <tbody>
    <row>
     <entry>funcid</entry>
     <entry><type>oid</></entry>
     <entry>The OID of the function</entry>
    </row>
    <row>
     <entry>schemaname</entry>
     <entry><type>name</></entry>
     <entry>The name of the schema the function is in</entry>
    </row>
    <row>
     <entry>funcname</entry>
     <entry><type>name</></entry>
     <entry>The name of the function</entry>
    </row>
    <row>
     <entry>calls</entry>
     <entry><type>bigint</></entry>
     <entry>Number of times the function has been called.
     This value can also be returned by directly calling
     the <function>pg_stat_get_function_calls</function> function.</entry>
    </row>
    <row>
     <entry>total_time</entry>
     <entry><type>bigint</></entry>
     <entry>Total time spent in this functions and all other functions
     called by it, in milliseconds.
     This value can also be returned by directly calling
     the <function>pg_stat_get_function_time</function> function.</entry>
    </row>
    <row>
     <entry>self_time</entry>
     <entry><type>bigint</></entry>
     <entry>Total time spent in this functions itself but not including
     other functions called by it, in milliseconds.
     This value can also be returned by directly calling
     the <function>pg_stat_get_function_self_time</function> function.</entry>
    </row>
   </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_stat_user_functions</structname> view will contain
   one row for each tracked function (as specified by the
   <xref linkend="guc-track-functions"> parameter).
  </para>

 <sect3 id="monitoring-stats-functions">
  <title>Other Statistics Functions</title>


  <table id="monitoring-stats-funcs-table">
   <title>Other 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 attached to the current session
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_get_activity</function>(<type>integer</type>)</literal></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_backend_idset()</function></literal></entry>
      <entry><type>setof integer</type></entry>
      <entry>
       Set of currently active server process numbers (from 1 to the
       number of active server processes).  See usage example in the text.
      </entry>
     </row>


     <row>
      <entry><literal><function>pg_stat_get_wal_senders()</function></literal></entry>
      <entry><type>setof record</type></entry>
      <entry>
       One record for each active wal sender. The fields returned are a subset
       of those in the <structname>pg_stat_replication</structname> view.
      </entry>
     </row>

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

     <row>
      <entry><literal><function>pg_stat_reset()</function></literal></entry>
      <entry><type>void</type></entry>
      <entry>
       Reset all statistics counters for the current database to zero
       (requires superuser privileges)
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_reset_shared</function>(text)</literal></entry>
      <entry><type>void</type></entry>
      <entry>
       Reset some of the shared statistics counters for the database cluster to
       zero (requires superuser privileges).  Calling
       <literal>pg_stat_reset_shared('bgwriter')</> will zero all the values shown by
       <structname>pg_stat_bgwriter</>.
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_reset_single_table_counters</function>(oid)</literal></entry>
      <entry><type>void</type></entry>
      <entry>
       Reset statistics for a single table or index in the current database to
       zero (requires superuser privileges)
      </entry>
     </row>

     <row>
      <entry><literal><function>pg_stat_reset_single_function_counters</function>(oid)</literal></entry>
      <entry><type>void</type></entry>
      <entry>
       Reset statistics for a single function in the current database to
       zero (requires superuser privileges)
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect3>
 </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="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, only the
   <ulink url="http://opensolaris.org/os/community/dtrace/">DTrace</ulink>
   utility is supported, which is available
   on OpenSolaris, Solaris 10, and Mac OS X Leopard. It is expected that
   DTrace will be available in the future on FreeBSD and possibly other
   operating systems.  The
   <ulink url="http://sourceware.org/systemtap/">SystemTap</ulink> project
   for Linux also provides a DTrace equivalent.  Supporting other dynamic
   tracing utilities is theoretically possible by changing the definitions for
   the macros in <filename>src/include/utils/probes.h</>.
  </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</> 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</>'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>transaction-start</entry>
     <entry>(LocalTransactionId)</entry>
     <entry>Probe that fires at the start of a new transaction.
      arg0 is the transaction ID.</entry>
    </row>
    <row>
     <entry>transaction-commit</entry>
     <entry>(LocalTransactionId)</entry>
     <entry>Probe that fires when a transaction completes successfully.
      arg0 is the transaction ID.</entry>
    </row>
    <row>
     <entry>transaction-abort</entry>
     <entry>(LocalTransactionId)</entry>
     <entry>Probe that fires when a transaction completes unsuccessfully.
      arg0 is the transaction ID.</entry>
    </row>
    <row>
     <entry>query-start</entry>
     <entry>(const char *)</entry>
     <entry>Probe that fires when the processing of a query is started.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry>query-done</entry>
     <entry>(const char *)</entry>
     <entry>Probe that fires when the processing of a query is complete.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry>query-parse-start</entry>
     <entry>(const char *)</entry>
     <entry>Probe that fires when the parsing of a query is started.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry>query-parse-done</entry>
     <entry>(const char *)</entry>
     <entry>Probe that fires when the parsing of a query is complete.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry>query-rewrite-start</entry>
     <entry>(const char *)</entry>
     <entry>Probe that fires when the rewriting of a query is started.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry>query-rewrite-done</entry>
     <entry>(const char *)</entry>
     <entry>Probe that fires when the rewriting of a query is complete.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry>query-plan-start</entry>
     <entry>()</entry>
     <entry>Probe that fires when the planning of a query is started.</entry>
    </row>
    <row>
     <entry>query-plan-done</entry>
     <entry>()</entry>
     <entry>Probe that fires when the planning of a query is complete.</entry>
    </row>
    <row>
     <entry>query-execute-start</entry>
     <entry>()</entry>
     <entry>Probe that fires when the execution of a query is started.</entry>
    </row>
    <row>
     <entry>query-execute-done</entry>
     <entry>()</entry>
     <entry>Probe that fires when the execution of a query is complete.</entry>
    </row>
    <row>
     <entry>statement-status</entry>
     <entry>(const char *)</entry>
     <entry>Probe that fires anytime the server process updates its
      <structname>pg_stat_activity</>.<structfield>status</>.
      arg0 is the new status string.</entry>
    </row>
    <row>
     <entry>checkpoint-start</entry>
     <entry>(int)</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>checkpoint-done</entry>
     <entry>(int, int, int, int, int)</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 xlog file(s) added,
      removed and recycled respectively.</entry>
    </row>
    <row>
     <entry>clog-checkpoint-start</entry>
     <entry>(bool)</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>clog-checkpoint-done</entry>
     <entry>(bool)</entry>
     <entry>Probe that fires when the CLOG portion of a checkpoint is
      complete. arg0 has the same meaning as for clog-checkpoint-start.</entry>
    </row>
    <row>
     <entry>subtrans-checkpoint-start</entry>
     <entry>(bool)</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>subtrans-checkpoint-done</entry>
     <entry>(bool)</entry>
     <entry>Probe that fires when the SUBTRANS portion of a checkpoint is
      complete. arg0 has the same meaning as for
      subtrans-checkpoint-start.</entry>
    </row>
    <row>
     <entry>multixact-checkpoint-start</entry>
     <entry>(bool)</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>multixact-checkpoint-done</entry>
     <entry>(bool)</entry>
     <entry>Probe that fires when the MultiXact portion of a checkpoint is
      complete. arg0 has the same meaning as for
      multixact-checkpoint-start.</entry>
    </row>
    <row>
     <entry>buffer-checkpoint-start</entry>
     <entry>(int)</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>buffer-sync-start</entry>
     <entry>(int, int)</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>buffer-sync-written</entry>
     <entry>(int)</entry>
     <entry>Probe that fires after each buffer is written during checkpoint.
      arg0 is the ID number of the buffer.</entry>
    </row>
    <row>
     <entry>buffer-sync-done</entry>
     <entry>(int, int, int)</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
      buffer-sync-start); any difference reflects other processes flushing
      buffers during the checkpoint.</entry>
    </row>
    <row>
     <entry>buffer-checkpoint-sync-start</entry>
     <entry>()</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>buffer-checkpoint-done</entry>
     <entry>()</entry>
     <entry>Probe that fires when syncing of buffers to disk is
      complete.</entry>
    </row>
    <row>
     <entry>twophase-checkpoint-start</entry>
     <entry>()</entry>
     <entry>Probe that fires when the two-phase portion of a checkpoint is
      started.</entry>
    </row>
    <row>
     <entry>twophase-checkpoint-done</entry>
     <entry>()</entry>
     <entry>Probe that fires when the two-phase portion of a checkpoint is
      complete.</entry>
    </row>
    <row>
     <entry>buffer-read-start</entry>
     <entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool)</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 InvalidBackendId (-1) for a shared buffer.
      arg6 is true for a relation extension request, false for normal
      read.</entry>
    </row>
    <row>
     <entry>buffer-read-done</entry>
     <entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool, bool)</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 InvalidBackendId (-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>buffer-flush-start</entry>
     <entry>(ForkNumber, BlockNumber, Oid, Oid, Oid)</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>buffer-flush-done</entry>
     <entry>(ForkNumber, BlockNumber, Oid, Oid, Oid)</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 buffer-flush-start.</entry>
    </row>
    <row>
     <entry>buffer-write-dirty-start</entry>
     <entry>(ForkNumber, BlockNumber, Oid, Oid, Oid)</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 bgwriter 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>buffer-write-dirty-done</entry>
     <entry>(ForkNumber, BlockNumber, Oid, Oid, Oid)</entry>
     <entry>Probe that fires when a dirty-buffer write is complete.
      The arguments are the same as for buffer-write-dirty-start.</entry>
    </row>
    <row>
     <entry>wal-buffer-write-dirty-start</entry>
     <entry>()</entry>
     <entry>Probe that fires when 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>wal-buffer-write-dirty-done</entry>
     <entry>()</entry>
     <entry>Probe that fires when a dirty WAL buffer write is complete.</entry>
    </row>
    <row>
     <entry>xlog-insert</entry>
     <entry>(unsigned char, unsigned char)</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>xlog-switch</entry>
     <entry>()</entry>
     <entry>Probe that fires when a WAL segment switch is requested.</entry>
    </row>
    <row>
     <entry>smgr-md-read-start</entry>
     <entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</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 InvalidBackendId (-1) for a shared buffer.</entry>
    </row>
    <row>
     <entry>smgr-md-read-done</entry>
     <entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</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 InvalidBackendId (-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>smgr-md-write-start</entry>
     <entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</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 InvalidBackendId (-1) for a shared buffer.</entry>
    </row>
    <row>
     <entry>smgr-md-write-done</entry>
     <entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</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 InvalidBackendId (-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>sort-start</entry>
     <entry>(int, bool, int, int, bool)</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.</entry>
    </row>
    <row>
     <entry>sort-done</entry>
     <entry>(bool, long)</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>lwlock-acquire</entry>
     <entry>(LWLockId, LWLockMode)</entry>
     <entry>Probe that fires when an LWLock has been acquired.
      arg0 is the LWLock's ID.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry>lwlock-release</entry>
     <entry>(LWLockId)</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 ID.</entry>
    </row>
    <row>
     <entry>lwlock-wait-start</entry>
     <entry>(LWLockId, LWLockMode)</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 ID.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry>lwlock-wait-done</entry>
     <entry>(LWLockId, LWLockMode)</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 ID.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry>lwlock-condacquire</entry>
     <entry>(LWLockId, LWLockMode)</entry>
     <entry>Probe that fires when an LWLock was successfully acquired when the
      caller specified no waiting.
      arg0 is the LWLock's ID.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry>lwlock-condacquire-fail</entry>
     <entry>(LWLockId, LWLockMode)</entry>
     <entry>Probe that fires when an LWLock was not successfully acquired when
      the caller specified no waiting.
      arg0 is the LWLock's ID.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry>lock-wait-start</entry>
     <entry>(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</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>lock-wait-done</entry>
     <entry>(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</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 lock-wait-start.</entry>
    </row>
    <row>
     <entry>deadlock-found</entry>
     <entry>()</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>LocalTransactionId</entry>
     <entry>unsigned int</entry>
    </row>
    <row>
     <entry>LWLockId</entry>
     <entry>int</entry>
    </row>
    <row>
     <entry>LWLockMode</entry>
     <entry>int</entry>
    </row>
    <row>
     <entry>LOCKMODE</entry>
     <entry>int</entry>
    </row>
    <row>
     <entry>BlockNumber</entry>
     <entry>unsigned int</entry>
    </row>
    <row>
     <entry>Oid</entry>
     <entry>unsigned int</entry>
    </row>
    <row>
     <entry>ForkNumber</entry>
     <entry>int</entry>
    </row>
    <row>
     <entry>bool</entry>
     <entry>char</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</> 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>

  <para>
   More example scripts can be found in the PgFoundry
   <ulink url="http://pgfoundry.org/projects/dtrace/">dtrace project</ulink>.
  </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</>
    </para>
   </step>

   <step>
    <para>
     Include <filename>pg_trace.h</> if it is not already present in the
     module(s) containing the probe points, and insert
     <literal>TRACE_POSTGRESQL</> 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</> and
     requires a parameter of type LocalTransactionId
    </para>
   </step>

   <step>
    <para>
     Add the probe definition to <filename>src/backend/utils/probes.d</>:
<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</> is the name to document for
     users.
    </para>
   </step>

   <step>
    <para>
     At compile time, <literal>transaction__start</> is converted to a macro
     called <literal>TRACE_POSTGRESQL_TRANSACTION_START</> (notice the
     underscores are single here), which is available by including
     <filename>pg_trace.h</>.  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</>, the arguments to a trace
      macro will be evaluated whenever control passes through the
      macro, <emphasis>even if no tracing is being done</>.  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</> macro.
     </para>
    </listitem>
   </itemizedlist>

  </para>

  </sect2>

 </sect1>

</chapter>