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buffers that cannot possibly need to be cleaned, and estimates how many buffers it should try to clean based on moving averages of recent allocation requests and density of reusable buffers. The patch also adds a couple more columns to pg_stat_bgwriter to help measure the effectiveness of the bgwriter. Greg Smith, building on his own work and ideas from several other people, in particular a much older patch from Itagaki Takahiro.
1196 lines
42 KiB
Plaintext
1196 lines
42 KiB
Plaintext
<!-- $PostgreSQL: pgsql/doc/src/sgml/monitoring.sgml,v 1.54 2007/09/25 20:03:37 tgl Exp $ -->
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<chapter id="monitoring">
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<title>Monitoring Database Activity</title>
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<indexterm zone="monitoring">
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<primary>monitoring</primary>
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<secondary>database activity</secondary>
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</indexterm>
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<indexterm zone="monitoring">
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<primary>database activity</primary>
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<secondary>monitoring</secondary>
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</indexterm>
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<para>
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A database administrator frequently wonders, <quote>What is the system
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doing right now?</quote>
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This chapter discusses how to find that out.
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</para>
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<para>
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Several tools are available for monitoring database activity and
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analyzing performance. Most of this chapter is devoted to describing
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<productname>PostgreSQL</productname>'s statistics collector,
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but one should not neglect regular Unix monitoring programs such as
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<command>ps</>, <command>top</>, <command>iostat</>, and <command>vmstat</>.
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Also, once one has identified a
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poorly-performing query, further investigation might be needed using
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<productname>PostgreSQL</productname>'s <xref linkend="sql-explain"
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endterm="sql-explain-title"> command.
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<xref linkend="using-explain"> discusses <command>EXPLAIN</>
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and other methods for understanding the behavior of an individual
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query.
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</para>
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<sect1 id="monitoring-ps">
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<title>Standard Unix Tools</Title>
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<indexterm zone="monitoring-ps">
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<primary>ps</primary>
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<secondary>to monitor activity</secondary>
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</indexterm>
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<para>
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On most platforms, <productname>PostgreSQL</productname> modifies its
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command title as reported by <command>ps</>, so that individual server
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processes can readily be identified. A sample display is
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<screen>
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$ ps auxww | grep ^postgres
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postgres 960 0.0 1.1 6104 1480 pts/1 SN 13:17 0:00 postgres -i
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postgres 963 0.0 1.1 7084 1472 pts/1 SN 13:17 0:00 postgres: writer process
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postgres 965 0.0 1.1 6152 1512 pts/1 SN 13:17 0:00 postgres: stats collector process
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postgres 998 0.0 2.3 6532 2992 pts/1 SN 13:18 0:00 postgres: tgl runbug 127.0.0.1 idle
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postgres 1003 0.0 2.4 6532 3128 pts/1 SN 13:19 0:00 postgres: tgl regression [local] SELECT waiting
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postgres 1016 0.1 2.4 6532 3080 pts/1 SN 13:19 0:00 postgres: tgl regression [local] idle in transaction
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</screen>
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(The appropriate invocation of <command>ps</> varies across different
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platforms, as do the details of what is shown. This example is from a
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recent Linux system.) The first process listed here is the
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master server process. The command arguments
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shown for it are the same ones given when it was launched. The next two
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processes are background worker processes automatically launched by the
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master process. (The <quote>stats collector</> process will not be present
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if you have set
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the system not to start the statistics collector.) Each of the remaining
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processes is a server process handling one client connection. Each such
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process sets its command line display in the form
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<screen>
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postgres: <replaceable>user</> <replaceable>database</> <replaceable>host</> <replaceable>activity</>
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</screen>
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The user, database, and connection source host items remain the same for
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the life of the client connection, but the activity indicator changes.
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The activity can be <literal>idle</> (i.e., waiting for a client command),
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<literal>idle in transaction</> (waiting for client inside a <command>BEGIN</> block),
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or a command type name such as <literal>SELECT</>. Also,
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<literal>waiting</> is attached if the server process is presently waiting
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on a lock held by another server process. In the above example we can infer
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that process 1003 is waiting for process 1016 to complete its transaction and
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thereby release some lock or other.
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</para>
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<para>
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If you have turned off <xref linkend="guc-update-process-title"> then the
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activity indicator is not updated; the process title is set only once
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when a new process is launched. On some platforms this saves a useful
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amount of per-command overhead, on others it's insignificant.
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</para>
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<tip>
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<para>
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<productname>Solaris</productname> requires special handling. You must
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use <command>/usr/ucb/ps</command>, rather than
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<command>/bin/ps</command>. You also must use two <option>w</option>
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flags, not just one. In addition, your original invocation of the
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<command>postgres</command> command must have a shorter
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<command>ps</command> status display than that provided by each
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server process. If you fail to do all three things, the <command>ps</>
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output for each server process will be the original <command>postgres</>
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command line.
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</para>
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</tip>
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</sect1>
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<sect1 id="monitoring-stats">
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<title>The Statistics Collector</Title>
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<indexterm zone="monitoring-stats">
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<primary>statistics</primary>
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</indexterm>
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<para>
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<productname>PostgreSQL</productname>'s <firstterm>statistics collector</>
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is a subsystem that supports collection and reporting of information about
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server activity. Presently, the collector can count accesses to tables
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and indexes in both disk-block and individual-row terms. It also tracks
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total numbers of rows in each table, and the last vacuum and analyze times
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for each table.
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</para>
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<para>
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<productname>PostgreSQL</productname> also supports determining the exact
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command currently being executed by other server processes. This is an
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independent facility that does not depend on the collector process.
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</para>
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<sect2 id="monitoring-stats-setup">
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<title>Statistics Collection Configuration</Title>
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<para>
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Since collection of statistics adds some overhead to query execution,
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the system can be configured to collect or not collect information.
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This is controlled by configuration parameters that are normally set in
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<filename>postgresql.conf</>. (See <xref linkend="runtime-config"> for
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details about setting configuration parameters.)
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</para>
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<para>
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The parameter <xref linkend="guc-track-counts"> controls whether
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information is actually sent to the collector process and thus determines
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whether any run-time overhead occurs for event counting.
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</para>
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<para>
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The parameter <xref linkend="guc-track-activities"> enables monitoring
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of the current command being executed by any server process.
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</para>
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<para>
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Normally these parameters are set in <filename>postgresql.conf</> so
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that they apply to all server processes, but it is possible to turn
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them on or off in individual sessions using the <xref
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linkend="sql-set" endterm="sql-set-title"> command. (To prevent
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ordinary users from hiding their activity from the administrator,
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only superusers are allowed to change these parameters with
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<command>SET</>.)
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</para>
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</sect2>
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<sect2 id="monitoring-stats-views">
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<title>Viewing Collected Statistics</Title>
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<para>
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Several predefined views, listed in <xref
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linkend="monitoring-stats-views-table">, are available to show the results
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of statistics collection. Alternatively, one can
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build custom views using the underlying statistics functions.
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</para>
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<para>
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When using the statistics to monitor current activity, it is important
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to realize that the information does not update instantaneously.
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Each individual server process transmits new statistical counts to
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the collector just before going idle; so a query or transaction still in
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progress does not affect the displayed totals. Also, the collector itself
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emits a new report at most once per <varname>PGSTAT_STAT_INTERVAL</varname>
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milliseconds (500 unless altered while building the server). So the
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displayed information lags behind actual activity. However, current-query
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information collected by <varname>track_activities</varname> is
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always up-to-date.
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</para>
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<para>
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Another important point is that when a server process is asked to display
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any of these statistics, it first fetches the most recent report emitted by
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the collector process and then continues to use this snapshot for all
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statistical views and functions until the end of its current transaction.
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So the statistics will appear not to change as long as you continue the
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current transaction. Similarly, information about the current queries of
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all processes is collected when any such information is first requested
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within a transaction, and the same information will be displayed throughout
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the transaction.
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This is a feature, not a bug, because it allows you to perform several
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queries on the statistics and correlate the results without worrying that
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the numbers are changing underneath you. But if you want to see new
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results with each query, be sure to do the queries outside any transaction
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block. Alternatively, you can invoke
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<function>pg_stat_clear_snapshot</function>(), which will discard the
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current transaction's statistics snapshot (if any). The next use of
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statistical information will cause a new snapshot to be fetched.
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</para>
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<table id="monitoring-stats-views-table">
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<title>Standard Statistics Views</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>View Name</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><structname>pg_stat_activity</></entry>
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<entry>One row per server process, showing database OID, database
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name, process <acronym>ID</>, user OID, user name, current query,
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query's waiting status, time at which the current transaction and
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current query began execution, time at which the process was
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started, and client's address and port number. The columns that
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report data on the current query are available unless the parameter
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<varname>track_activities</varname> has been turned off.
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Furthermore, these columns are only visible if the user examining
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the view is a superuser or the same as the user owning the process
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being reported on.
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</entry>
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</row>
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<row>
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<entry><structname>pg_stat_bgwriter</></entry>
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<entry>One row only, showing cluster-wide statistics from the
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background writer: number of scheduled checkpoints, requested
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checkpoints, buffers written by checkpoints and cleaning scans,
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and the number of times the bgwriter stopped a cleaning scan
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because it had written too many buffers. Also includes
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statistics about the shared buffer pool, including buffers written
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by backends (that is, not by the background writer) and total buffers
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allocated.
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</entry>
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</row>
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<row>
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<entry><structname>pg_stat_database</></entry>
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<entry>One row per database, showing database OID, database name,
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number of active server processes connected to that database,
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number of transactions committed and rolled back in that database,
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total disk blocks read, total buffer hits (i.e., block
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read requests avoided by finding the block already in buffer cache),
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number of rows returned, inserted, updated and deleted.
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</entry>
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</row>
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<row>
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<entry><structname>pg_stat_all_tables</></entry>
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<entry>For each table in the current database (including TOAST tables),
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the table OID, schema and table name, number of sequential
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scans initiated, number of live rows fetched by sequential
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scans, number of index scans initiated (over all indexes
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belonging to the table), number of live rows fetched by index
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scans, numbers of row insertions, updates, and deletions,
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number of row updates that were HOT (i.e., no separate index update),
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numbers of live and dead rows,
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the last time the table was vacuumed manually,
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the last time it was vacuumed by the autovacuum daemon,
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the last time it was analyzed manually,
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and the last time it was analyzed by the autovacuum daemon.
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</entry>
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</row>
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<row>
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<entry><structname>pg_stat_sys_tables</></entry>
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<entry>Same as <structname>pg_stat_all_tables</>, except that only
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system tables are shown.</entry>
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</row>
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<row>
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<entry><structname>pg_stat_user_tables</></entry>
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<entry>Same as <structname>pg_stat_all_tables</>, except that only user
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tables are shown.</entry>
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</row>
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<row>
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<entry><structname>pg_stat_all_indexes</></entry>
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<entry>For each index in the current database,
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the table and index OID, schema, table and index name,
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number of index scans initiated on that index, number of
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index entries returned by index scans, and number of live table rows
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fetched by simple index scans using that index.
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</entry>
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</row>
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<row>
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<entry><structname>pg_stat_sys_indexes</></entry>
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<entry>Same as <structname>pg_stat_all_indexes</>, except that only
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indexes on system tables are shown.</entry>
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</row>
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<row>
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<entry><structname>pg_stat_user_indexes</></entry>
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<entry>Same as <structname>pg_stat_all_indexes</>, except that only
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indexes on user tables are shown.</entry>
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</row>
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<row>
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<entry><structname>pg_statio_all_tables</></entry>
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<entry>For each table in the current database (including TOAST tables),
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the table OID, schema and table name, number of disk
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blocks read from that table, number of buffer hits, numbers of
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disk blocks read and buffer hits in all indexes of that table,
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numbers of disk blocks read and buffer hits from that table's
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auxiliary TOAST table (if any), and numbers of disk blocks read
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and buffer hits for the TOAST table's index.
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</entry>
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</row>
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<row>
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<entry><structname>pg_statio_sys_tables</></entry>
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<entry>Same as <structname>pg_statio_all_tables</>, except that only
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system tables are shown.</entry>
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</row>
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<row>
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<entry><structname>pg_statio_user_tables</></entry>
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<entry>Same as <structname>pg_statio_all_tables</>, except that only
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user tables are shown.</entry>
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</row>
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<row>
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<entry><structname>pg_statio_all_indexes</></entry>
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<entry>For each index in the current database,
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the table and index OID, schema, table and index name,
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numbers of disk blocks read and buffer hits in that index.
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</entry>
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</row>
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<row>
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<entry><structname>pg_statio_sys_indexes</></entry>
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<entry>Same as <structname>pg_statio_all_indexes</>, except that only
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indexes on system tables are shown.</entry>
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</row>
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<row>
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<entry><structname>pg_statio_user_indexes</></entry>
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<entry>Same as <structname>pg_statio_all_indexes</>, except that only
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indexes on user tables are shown.</entry>
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</row>
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<row>
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<entry><structname>pg_statio_all_sequences</></entry>
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<entry>For each sequence object in the current database,
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the sequence OID, schema and sequence name,
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numbers of disk blocks read and buffer hits in that sequence.
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</entry>
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</row>
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<row>
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<entry><structname>pg_statio_sys_sequences</></entry>
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<entry>Same as <structname>pg_statio_all_sequences</>, except that only
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system sequences are shown. (Presently, no system sequences are defined,
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so this view is always empty.)</entry>
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</row>
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<row>
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<entry><structname>pg_statio_user_sequences</></entry>
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<entry>Same as <structname>pg_statio_all_sequences</>, except that only
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user sequences are shown.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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The per-index statistics are particularly useful to determine which
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indexes are being used and how effective they are.
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</para>
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<para>
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Beginning in <productname>PostgreSQL</productname> 8.1, indexes can be
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used either directly or via <quote>bitmap scans</>. In a bitmap scan
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the output of several indexes can be combined via AND or OR rules;
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so it is difficult to associate individual heap row fetches
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with specific indexes when a bitmap scan is used. Therefore, a bitmap
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scan increments the
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<structname>pg_stat_all_indexes</>.<structfield>idx_tup_read</>
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count(s) for the index(es) it uses, and it increments the
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<structname>pg_stat_all_tables</>.<structfield>idx_tup_fetch</>
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count for the table, but it does not affect
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<structname>pg_stat_all_indexes</>.<structfield>idx_tup_fetch</>.
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</para>
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|
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<note>
|
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<para>
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Before <productname>PostgreSQL</productname> 8.1, the
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<structfield>idx_tup_read</> and <structfield>idx_tup_fetch</> counts
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were essentially always equal. Now they can be different even without
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considering bitmap scans, because <structfield>idx_tup_read</> counts
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index entries retrieved from the index while <structfield>idx_tup_fetch</>
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counts live rows fetched from the table; the latter will be less if any
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dead or not-yet-committed rows are fetched using the index.
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</para>
|
|
</note>
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<para>
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The <structname>pg_statio_</> views are primarily useful to
|
|
determine the effectiveness of the buffer cache. When the number
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of actual disk reads is much smaller than the number of buffer
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hits, then the cache is satisfying most read requests without
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invoking a kernel call. However, these statistics do not give the
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entire story: due to the way in which <productname>PostgreSQL</>
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handles disk I/O, data that is not in the
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<productname>PostgreSQL</> buffer cache might still reside in the
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kernel's I/O cache, and might therefore still be fetched without
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requiring a physical read. Users interested in obtaining more
|
|
detailed information on <productname>PostgreSQL</> I/O behavior are
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advised to use the <productname>PostgreSQL</> statistics collector
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|
in combination with operating system utilities that allow insight
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|
into the kernel's handling of I/O.
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|
</para>
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|
|
<para>
|
|
Other ways of looking at the statistics can be set up by writing
|
|
queries that use the same underlying statistics access functions as
|
|
these standard views do. These functions are listed in <xref
|
|
linkend="monitoring-stats-funcs-table">. The per-database access
|
|
functions take a database OID as argument to identify which
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|
database to report on. The per-table and per-index functions take
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a table or index OID. (Note that only tables and indexes in the
|
|
current database can be seen with these functions.) The
|
|
per-server-process access functions take a server process
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number, which ranges from one to the number of currently active
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server processes.
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|
</para>
|
|
|
|
<table id="monitoring-stats-funcs-table">
|
|
<title>Statistics Access Functions</title>
|
|
|
|
<tgroup cols="3">
|
|
<thead>
|
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<row>
|
|
<entry>Function</entry>
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<entry>Return Type</entry>
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<entry>Description</entry>
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</row>
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</thead>
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|
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<tbody>
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<row>
|
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<entry><literal><function>pg_stat_get_db_numbackends</function>(<type>oid</type>)</literal></entry>
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<entry><type>integer</type></entry>
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<entry>
|
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Number of active server processes for database
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</entry>
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</row>
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<row>
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<entry><literal><function>pg_stat_get_db_xact_commit</function>(<type>oid</type>)</literal></entry>
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<entry><type>bigint</type></entry>
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<entry>
|
|
Transactions committed in database
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_db_xact_rollback</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Transactions rolled back in database
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_db_blocks_fetched</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of disk block fetch requests for database
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_db_blocks_hit</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of disk block fetch requests found in cache for database
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_db_tuples_returned</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of tuples returned for database
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_db_tuples_fetched</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of tuples fetched for database
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_db_tuples_inserted</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of tuples inserted in database
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_db_tuples_updated</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of tuples updated in database
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_db_tuples_deleted</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of tuples deleted in database
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_numscans</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of sequential scans done when argument is a table,
|
|
or number of index scans done when argument is an index
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_tuples_returned</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of rows read by sequential scans when argument is a table,
|
|
or number of index entries returned when argument is an index
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_tuples_fetched</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of table rows fetched by bitmap scans when argument is a table,
|
|
or table rows fetched by simple index scans using the index
|
|
when argument is an index
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_tuples_inserted</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of rows inserted into table
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_tuples_updated</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of rows updated in table (includes HOT updates)
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_tuples_deleted</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of rows deleted from table
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_tuples_hot_updated</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of rows HOT-updated in table
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_live_tuples</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of live rows in table
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_dead_tuples</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of dead rows in table
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_blocks_fetched</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of disk block fetch requests for table or index
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_blocks_hit</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
Number of disk block requests found in cache for table or index
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_last_vacuum_time</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>timestamptz</type></entry>
|
|
<entry>
|
|
Time of the last vacuum initiated by the user on this table
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_last_autovacuum_time</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>timestamptz</type></entry>
|
|
<entry>
|
|
Time of the last vacuum initiated by the autovacuum daemon on this table
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_last_analyze_time</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>timestamptz</type></entry>
|
|
<entry>
|
|
Time of the last analyze initiated by the user on this table
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_last_autoanalyze_time</function>(<type>oid</type>)</literal></entry>
|
|
<entry><type>timestamptz</type></entry>
|
|
<entry>
|
|
Time of the last analyze initiated by the autovacuum daemon on this
|
|
table
|
|
</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_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_backend_pid</function>(<type>integer</type>)</literal></entry>
|
|
<entry><type>integer</type></entry>
|
|
<entry>
|
|
Process ID of the given server process
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_backend_dbid</function>(<type>integer</type>)</literal></entry>
|
|
<entry><type>oid</type></entry>
|
|
<entry>
|
|
Database ID of the given server process
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_backend_userid</function>(<type>integer</type>)</literal></entry>
|
|
<entry><type>oid</type></entry>
|
|
<entry>
|
|
User ID of the given server process
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_backend_activity</function>(<type>integer</type>)</literal></entry>
|
|
<entry><type>text</type></entry>
|
|
<entry>
|
|
Active command of the given server process, but only if the
|
|
current user is a superuser or the same user as that of
|
|
the session being queried (and
|
|
<varname>track_activities</varname> is on)
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_backend_waiting</function>(<type>integer</type>)</literal></entry>
|
|
<entry><type>boolean</type></entry>
|
|
<entry>
|
|
True if the given server process is waiting for a lock,
|
|
but only if the current user is a superuser or the same user as that of
|
|
the session being queried (and
|
|
<varname>track_activities</varname> is on)
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_backend_activity_start</function>(<type>integer</type>)</literal></entry>
|
|
<entry><type>timestamp with time zone</type></entry>
|
|
<entry>
|
|
The time at which the given server process' currently
|
|
executing query was started, but only if the
|
|
current user is a superuser or the same user as that of
|
|
the session being queried (and
|
|
<varname>track_activities</varname> is on)
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_backend_xact_start</function>(<type>integer</type>)</literal></entry>
|
|
<entry><type>timestamp with time zone</type></entry>
|
|
<entry>
|
|
The time at which the given server process' currently
|
|
executing transaction was started, but only if the
|
|
current user is a superuser or the same user as that of
|
|
the session being queried (and
|
|
<varname>track_activities</varname> is on)
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_backend_start</function>(<type>integer</type>)</literal></entry>
|
|
<entry><type>timestamp with time zone</type></entry>
|
|
<entry>
|
|
The time at which the given server process was started, or
|
|
null if the current user is not a superuser nor the same user
|
|
as that of the session being queried
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_backend_client_addr</function>(<type>integer</type>)</literal></entry>
|
|
<entry><type>inet</type></entry>
|
|
<entry>
|
|
The IP address of the client connected to the given
|
|
server process. Null if the connection is over a Unix domain
|
|
socket. Also null if the current user is not a superuser nor
|
|
the same user as that of the session being queried
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_backend_client_port</function>(<type>integer</type>)</literal></entry>
|
|
<entry><type>integer</type></entry>
|
|
<entry>
|
|
The IP port number of the client connected to the given
|
|
server process. -1 if the connection is over a Unix domain
|
|
socket. Null if the current user is not a superuser nor the
|
|
same user as that of the session being queried
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_bgwriter_timed_checkpoints</function>()</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
The number of times the bgwriter has started timed checkpoints
|
|
(because the <varname>checkpoint_timeout</varname> time has expired)
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_bgwriter_requested_checkpoints</function>()</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
The number of times the bgwriter has started checkpoints based
|
|
on requests from backends because the <varname>checkpoint_segments</varname>
|
|
has been exceeded or because the <command>CHECKPOINT</command>
|
|
command has been issued
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_bgwriter_buf_written_checkpoints</function>()</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
The number of buffers written by the bgwriter during checkpoints
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_bgwriter_buf_written_clean</function>()</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
The number of buffers written by the bgwriter for routine cleaning of
|
|
dirty pages
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_bgwriter_maxwritten_clean</function>()</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
The number of times the bgwriter has stopped its cleaning scan because
|
|
it has written more buffers than specified in the
|
|
<varname>bgwriter_lru_maxpages</varname> parameter
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_buf_written_backend</function>()</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
The number of buffers written by backends because they needed
|
|
to allocate a new buffer
|
|
</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><literal><function>pg_stat_get_buf_alloc</function>()</literal></entry>
|
|
<entry><type>bigint</type></entry>
|
|
<entry>
|
|
The total number of buffer allocations
|
|
</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>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
<note>
|
|
<para>
|
|
<function>blocks_fetched</function> minus
|
|
<function>blocks_hit</function> gives the number of kernel
|
|
<function>read()</> calls issued for the table, index, or
|
|
database; but the actual number of physical reads is usually
|
|
lower due to kernel-level buffering.
|
|
</para>
|
|
</note>
|
|
|
|
<para>
|
|
The function <function>pg_stat_get_backend_idset</function> provides
|
|
a convenient way to generate one row for each active 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 procpid,
|
|
pg_stat_get_backend_activity(s.backendid) AS current_query
|
|
FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS s;
|
|
</programlisting>
|
|
</para>
|
|
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="monitoring-locks">
|
|
<title>Viewing Locks</title>
|
|
|
|
<indexterm zone="monitoring-locks">
|
|
<primary>lock</primary>
|
|
<secondary>monitoring</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
Another useful tool for monitoring database activity is the
|
|
<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. Currently, this facility is primarily intended for use by
|
|
database developers, as it requires substantial familiarity with the code.
|
|
</para>
|
|
|
|
<para>
|
|
A number of trace points, often called probes, are already inserted
|
|
into the source code. By default these probes are disabled, and
|
|
the user needs to explicitly tell the configure script to make the
|
|
probes available in <productname>PostgreSQL</productname>.
|
|
</para>
|
|
|
|
<para>
|
|
Currently, only the DTrace utility is supported, which is only available
|
|
on Solaris Express and Solaris 10+. It is expected that DTrace will
|
|
be available in the future on FreeBSD and Mac OS X.
|
|
Supporting other dynamic tracing utilities is theoretically possible by
|
|
changing the definitions for the <literal>PG_TRACE</> macros in
|
|
<filename>src/include/pg_trace.h</>.
|
|
</para>
|
|
|
|
<sect2 id="compiling-for-trace">
|
|
<title>Compiling for Dynamic Tracing</title>
|
|
|
|
<para>
|
|
By default, trace points are disabled, 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">
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<title>Built-in Trace Points</title>
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<para>
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A few standard trace points are provided in the source code
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(of course, more can be added as needed for a particular problem).
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These are shown in <xref linkend="trace-point-table">.
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</para>
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<table id="trace-point-table">
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<title>Built-in Trace Points</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Name</entry>
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<entry>Parameters</entry>
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<entry>Overview</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>transaction__start</entry>
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<entry>(int transactionId)</entry>
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<entry>The start of a new transaction.</entry>
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</row>
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<row>
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<entry>transaction__commit</entry>
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<entry>(int transactionId)</entry>
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<entry>The successful completion of a transaction.</entry>
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</row>
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<row>
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<entry>transaction__abort</entry>
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<entry>(int transactionId)</entry>
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<entry>The unsuccessful completion of a transaction.</entry>
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</row>
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<row>
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<entry>lwlock__acquire</entry>
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<entry>(int lockid, int mode)</entry>
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<entry>An LWLock has been acquired.</entry>
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</row>
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<row>
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<entry>lwlock__release</entry>
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<entry>(int lockid, int mode)</entry>
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<entry>An LWLock has been released.</entry>
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</row>
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<row>
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<entry>lwlock__startwait</entry>
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<entry>(int lockid, int mode)</entry>
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<entry>An LWLock was not immediately available and a backend
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has begun to wait for the lock to become available.
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</entry>
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</row>
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<row>
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<entry>lwlock__endwait</entry>
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<entry>(int lockid, int mode)</entry>
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<entry>A backend has been released from its wait for an LWLock.
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</entry>
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</row>
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<row>
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<entry>lwlock__condacquire</entry>
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<entry>(int lockid, int mode)</entry>
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<entry>An LWLock was successfully acquired when the caller specified no
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waiting.
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</entry>
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</row>
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<row>
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<entry>lwlock__condacquire__fail</entry>
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<entry>(int lockid, int mode)</entry>
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<entry>An LWLock was not successfully acquired when the caller specified
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no waiting.
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</entry>
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</row>
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<row>
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<entry>lock__startwait</entry>
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<entry>(int locktag_field2, int lockmode)</entry>
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<entry>A request for a heavyweight lock (lmgr lock) has begun to wait
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because the lock is not available.
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</entry>
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</row>
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<row>
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<entry>lock__endwait</entry>
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<entry>(int locktag_field2, int lockmode)</entry>
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<entry>A request for a heavyweight lock (lmgr lock) has finished waiting
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(i.e., has acquired the lock).
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</sect2>
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<sect2 id="using-trace-points">
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<title>Using Trace Points</title>
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<para>
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The example below shows a DTrace script for analyzing transaction
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counts on the system, as an alternative to snapshotting
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<structname>pg_stat_database</> before and after a performance test:
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<programlisting>
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#!/usr/sbin/dtrace -qs
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postgresql$1:::transaction-start
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{
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@start["Start"] = count();
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self->ts = timestamp;
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}
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postgresql$1:::transaction-abort
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{
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@abort["Abort"] = count();
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}
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postgresql$1:::transaction-commit
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/self->ts/
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{
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@commit["Commit"] = count();
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@time["Total time (ns)"] = sum(timestamp - self->ts);
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self->ts=0;
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}
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</programlisting>
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Note how the double underline in trace point names needs to
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be replaced by a hyphen when using D script.
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When executed, the example D script gives output such as:
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<screen>
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# ./txn_count.d `pgrep -n postgres`
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^C
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Start 71
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Commit 70
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Total time (ns) 2312105013
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</screen>
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</para>
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<para>
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You should remember that trace programs need to be carefully written and
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debugged prior to their use, otherwise the trace information collected might
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be meaningless. In most cases where problems are found it is the
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instrumentation that is at fault, not the underlying system. When
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discussing information found using dynamic tracing, be sure to enclose
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the script used to allow that too to be checked and discussed.
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</para>
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</sect2>
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<sect2 id="defining-trace-points">
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<title>Defining Trace Points</title>
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<para>
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New trace points can be defined within the code wherever the developer
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desires, though this will require a recompilation.
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</para>
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<para>
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A trace point can be inserted by using one of the
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trace macros. These are chosen according to how many variables will
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be made available for inspection at that trace point. Tracing the
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occurrence of an event can be achieved with a single line, using
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just the trace point name, e.g.:
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<programlisting>
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PG_TRACE (my__new__trace__point);
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</programlisting>
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More complex trace points can be provided with one or more variables
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|
for inspection by the dynamic tracing utility by using the
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<literal>PG_TRACE</><replaceable>n</> macro that corresponds to the number
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of parameters after the trace point name:
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<programlisting>
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|
PG_TRACE3 (my__complex__event, varX, varY, varZ);
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</programlisting>
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The definition of the transaction__start trace point is shown below:
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<programlisting>
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static void
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StartTransaction(void)
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{
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...
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/*
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* generate a new transaction id
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*/
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s->transactionId = GetNewTransactionId(false);
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XactLockTableInsert(s->transactionId);
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PG_TRACE1(transaction__start, s->transactionId);
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|
|
...
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}
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</programlisting>
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Note how the transaction ID is made available to the dynamic tracing
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|
utility.
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</para>
|
|
|
|
<para>
|
|
The dynamic tracing utility might require you to further define these trace
|
|
points. For example, DTrace requires you to add new probes to the file
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|
<filename>src/backend/utils/probes.d</> as shown here:
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|
<programlisting>
|
|
provider postgresql {
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...
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probe transaction__start(int);
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...
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};
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</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
You should take care that the data types specified for the probe arguments
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match the datatypes of the variables used in the <literal>PG_TRACE</>
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macro. This is not checked at compile time. You can check that your newly
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added trace point is available by recompiling, then running the new binary,
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and as root, executing a DTrace command such as:
|
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<programlisting>
|
|
dtrace -l -n transaction-start
|
|
</programlisting>
|
|
</para>
|
|
</sect2>
|
|
|
|
</sect1>
|
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|
|
</chapter>
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