database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-10-29 22:49:41 +03:00
Code Activity
Files
d5dd3d451e7d898ea2492d80539f74f032e3f408
postgres/doc/src/sgml/monitoring.sgml
Tom Lane cb8b6618ce Revise pgstats stuff to fix the problems with not counting accesses 
generated by bitmap index scans.  Along the way, simplify and speed up
the code for counting sequential and index scans; it was both confusing
and inefficient to be taking care of that in the per-tuple loops, IMHO.
initdb forced because of internal changes in pg_stat view definitions.
2005-10-06 02:29:23 +00:00

741 lines
28 KiB
Plaintext
Raw Blame History

<!--
$PostgreSQL: pgsql/doc/src/sgml/monitoring.sgml,v 1.29 2005/10/06 02:29:06 tgl Exp $
-->
<chapter id="monitoring">
<title>Monitoring Database Activity</title>
<indexterm zone="monitoring">
<primary>monitoring</primary>
<secondary>database activity</secondary>
</indexterm>
<indexterm zone="monitoring">
<primary>database activity</primary>
<secondary>monitoring</secondary>
</indexterm>
<para>
A database administrator frequently wonders, <quote>What is the system
doing right now?</quote>
This chapter discusses how to find that out.
</para>
<para>
Several tools are available for monitoring database activity and
analyzing performance. Most of this chapter is devoted to describing
<productname>PostgreSQL</productname>'s statistics collector,
but one should not neglect regular Unix monitoring programs such as
<command>ps</>, <command>top</>, <command>iostat</>, and <command>vmstat</>.
Also, once one has identified a
poorly-performing query, further investigation may be needed using
<productname>PostgreSQL</productname>'s <xref linkend="sql-explain"
endterm="sql-explain-title"> command.
<xref linkend="using-explain"> discusses <command>EXPLAIN</>
and other methods for understanding the behavior of an individual
query.
</para>
<sect1 id="monitoring-ps">
<title>Standard Unix Tools</Title>
<indexterm zone="monitoring-ps">
<primary>ps</primary>
<secondary>to monitor activity</secondary>
</indexterm>
<para>
On most platforms, <productname>PostgreSQL</productname> modifies its
command title as reported by <command>ps</>, so that individual server
processes can readily be identified. A sample display is
<screen>
$ ps auxww | grep ^postgres
postgres 960 0.0 1.1 6104 1480 pts/1 SN 13:17 0:00 postmaster -i
postgres 963 0.0 1.1 7084 1472 pts/1 SN 13:17 0:00 postgres: stats buffer process
postgres 965 0.0 1.1 6152 1512 pts/1 SN 13:17 0:00 postgres: stats collector process
postgres 998 0.0 2.3 6532 2992 pts/1 SN 13:18 0:00 postgres: tgl runbug 127.0.0.1 idle
postgres 1003 0.0 2.4 6532 3128 pts/1 SN 13:19 0:00 postgres: tgl regression [local] SELECT waiting
postgres 1016 0.1 2.4 6532 3080 pts/1 SN 13:19 0:00 postgres: tgl regression [local] idle in transaction
</screen>
(The appropriate invocation of <command>ps</> varies across different
platforms, as do the details of what is shown. This example is from a
recent Linux system.) The first process listed here is the
<application>postmaster</>, the master server process. The command arguments
shown for it are the same ones given when it was launched. The next two
processes implement the statistics collector, which will be described in
detail in the next section. (These will not be present if you have set
the system not to start the statistics collector.) Each of the remaining
processes is a server process handling one client connection. Each such
process sets its command line display in the form
<screen>
postgres: <replaceable>user</> <replaceable>database</> <replaceable>host</> <replaceable>activity</>
</screen>
The user, database, and connection source host items remain the same for
the life of the client connection, but the activity indicator changes.
The activity may be <literal>idle</> (i.e., waiting for a client command),
<literal>idle in transaction</> (waiting for client inside a <command>BEGIN</> block),
or a command type name such as <literal>SELECT</>. Also,
<literal>waiting</> is attached if the server process is presently waiting
on a lock held by another server process. In the above example we can infer
that process 1003 is waiting for process 1016 to complete its transaction and
thereby release some lock or other.
</para>
<tip>
<para>
<productname>Solaris</productname> requires special handling. You must
use <command>/usr/ucb/ps</command>, rather than
<command>/bin/ps</command>. You also must use two <option>w</option>
flags, not just one. In addition, your original invocation of the
<command>postmaster</command> command must have a shorter
<command>ps</command> status display than that provided by each
server process. If you fail to do all three things, the <command>ps</>
output for each server process will be the original <command>postmaster</>
command line.
</para>
</tip>
</sect1>
<sect1 id="monitoring-stats">
<title>The Statistics Collector</Title>
<indexterm zone="monitoring-stats">
<primary>statistics</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname>'s <firstterm>statistics collector</>
is a subsystem that supports collection and reporting of information about
server activity. Presently, the collector can count accesses to tables
and indexes in both disk-block and individual-row terms. It also supports
determining the exact command currently being executed by other server
processes.
</para>
<sect2 id="monitoring-stats-setup">
<title>Statistics Collection Configuration</Title>
<para>
Since collection of statistics adds some overhead to query execution,
the system can be configured to collect or not collect information.
This is controlled by configuration parameters that are normally set in
<filename>postgresql.conf</>. (See <xref linkend="runtime-config"> for
details about setting configuration parameters.)
</para>
<para>
The parameter <xref linkend="guc-stats-start-collector"> must be
set to <literal>true</> for the statistics collector to be launched
at all. This is the default and recommended setting, but it may be
turned off if you have no interest in statistics and want to
squeeze out every last drop of overhead. (The savings is likely to
be small, however.) Note that this option cannot be changed while
the server is running.
</para>
<para>
The parameters <xref linkend="guc-stats-command-string">,
<xref linkend="guc-stats-block-level">, and <xref
linkend="guc-stats-row-level"> control how much information is
actually sent to the collector and thus determine how much run-time
overhead occurs. These respectively determine whether a server
process sends its current command string, disk-block-level access
statistics, and row-level access statistics to the collector.
Normally these parameters are set in <filename>postgresql.conf</>
so that they apply to all server processes, but it is possible to
turn them on or off in individual sessions using the <xref
linkend="sql-set" endterm="sql-set-title"> command. (To prevent
ordinary users from hiding their activity from the administrator,
only superusers are allowed to change these parameters with
<command>SET</>.)
</para>
<note>
<para>
Since the parameters <varname>stats_command_string</varname>,
<varname>stats_block_level</varname>, and
<varname>stats_row_level</varname> default to <literal>false</>,
very few statistics are collected in the default
configuration. Enabling one or more of these configuration
variables will significantly enhance the amount of useful data
produced by the statistics collector, at the expense of
additional run-time overhead.
</para>
</note>
</sect2>
<sect2 id="monitoring-stats-views">
<title>Viewing Collected Statistics</Title>
<para>
Several predefined views, listed in <xref
linkend="monitoring-stats-views-table">, are available to show the results
of statistics collection. Alternatively, one can
build custom views using the underlying statistics functions.
</para>
<para>
When using the statistics to monitor current activity, it is important
to realize that the information does not update instantaneously.
Each individual server process transmits new block and row access counts to
the collector just before going idle; so a query or transaction still in
progress does not affect the displayed totals. Also, the collector itself
emits a new report at most once per <varname>PGSTAT_STAT_INTERVAL</varname>
milliseconds (500 unless altered while building the server). So the
displayed information lags behind actual activity. Current-query
information is reported to the collector immediately, but is still subject
to the <varname>PGSTAT_STAT_INTERVAL</varname> delay before it becomes
visible.
</para>
<para>
Another important point is that when a server process is asked to display
any of these statistics, it first fetches the most recent report emitted by
the collector process and then continues to use this snapshot for all
statistical views and functions until the end of its current transaction.
So the statistics will appear not to change as long as you continue the
current transaction.
This is a feature, not a bug, because it allows you to perform several
queries on the statistics and correlate the results without worrying that
the numbers are changing underneath you. But if you want to see new
results with each query, be sure to do the queries outside any transaction
block.
</para>
<table id="monitoring-stats-views-table">
<title>Standard Statistics Views</title>
<tgroup cols="2">
<thead>
<row>
<entry>View Name</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structname>pg_stat_activity</></entry>
<entry>One row per server process, showing database OID, database name,
process <acronym>ID</>, user OID, user name, current query, time at
which the current query began execution, time at which the process
was started, and client's address and port number. The columns
that report data on the current query are only available if the
parameter <varname>stats_command_string</varname> has been
turned on. Furthermore, these columns read as null unless the
user examining the view is a superuser or the same as the user
owning the process being reported on. (Note that because of the
collector's reporting delay, the current query will only be
up-to-date for long-running queries.)</entry>
</row>
<row>
<entry><structname>pg_stat_database</></entry>
<entry>One row per database, showing database OID, database name,
number of active server processes connected to that database,
number of transactions committed and rolled back in that database,
total disk blocks read, and total buffer hits (i.e., block
read requests avoided by finding the block already in buffer cache).
</entry>
</row>
<row>
<entry><structname>pg_stat_all_tables</></entry>
<entry>For each table in the current database (including TOAST tables),
the table OID, schema and table name, number of sequential
scans initiated, number of live rows fetched by sequential
scans, number of index scans initiated (over all indexes
belonging to the table), number of live rows fetched by index
scans,
and numbers of row insertions, updates, and deletions.</entry>
</row>
<row>
<entry><structname>pg_stat_sys_tables</></entry>
<entry>Same as <structname>pg_stat_all_tables</>, except that only
system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_user_tables</></entry>
<entry>Same as <structname>pg_stat_all_tables</>, except that only user
tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_all_indexes</></entry>
<entry>For each index in the current database,
the table and index OID, schema, table and index name,
number of index scans initiated on that index, number of
index entries returned by index scans, and number of live table rows
fetched by simple index scans using that index.
</entry>
</row>
<row>
<entry><structname>pg_stat_sys_indexes</></entry>
<entry>Same as <structname>pg_stat_all_indexes</>, except that only
indexes on system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_user_indexes</></entry>
<entry>Same as <structname>pg_stat_all_indexes</>, except that only
indexes on user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_tables</></entry>
<entry>For each table in the current database (including TOAST tables),
the table OID, schema and table name, number of disk
blocks read from that table, number of buffer hits, numbers of
disk blocks read and buffer hits in all indexes of that table,
numbers of disk blocks read and buffer hits from that table's
auxiliary TOAST table (if any), and numbers of disk blocks read
and buffer hits for the TOAST table's index.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_tables</></entry>
<entry>Same as <structname>pg_statio_all_tables</>, except that only
system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_user_tables</></entry>
<entry>Same as <structname>pg_statio_all_tables</>, except that only
user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_indexes</></entry>
<entry>For each index in the current database,
the table and index OID, schema, table and index name,
numbers of disk blocks read and buffer hits in that index.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_indexes</></entry>
<entry>Same as <structname>pg_statio_all_indexes</>, except that only
indexes on system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_user_indexes</></entry>
<entry>Same as <structname>pg_statio_all_indexes</>, except that only
indexes on user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_sequences</></entry>
<entry>For each sequence object in the current database,
the sequence OID, schema and sequence name,
numbers of disk blocks read and buffer hits in that sequence.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_sequences</></entry>
<entry>Same as <structname>pg_statio_all_sequences</>, except that only
system sequences are shown. (Presently, no system sequences are defined,
so this view is always empty.)</entry>
</row>
<row>
<entry><structname>pg_statio_user_sequences</></entry>
<entry>Same as <structname>pg_statio_all_sequences</>, except that only
user sequences are shown.</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The per-index statistics are particularly useful to determine which
indexes are being used and how effective they are.
</para>
<para>
Beginning in <productname>PostgreSQL</productname> 8.1, indexes can be
used either directly or via <quote>bitmap scans</>. In a bitmap scan
the output of several indexes can be combined via AND or OR rules;
so it is difficult to associate individual heap row fetches
with specific indexes when a bitmap scan is used. Therefore, a bitmap
scan increments the
<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.
</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 may still reside in the
kernel's I/O cache, and may 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 argument to identify which
database to report on. The per-table and per-index functions take
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
number, which ranges from one to the number of currently active
server processes.
</para>
<table id="monitoring-stats-funcs-table">
<title>Statistics Access Functions</title>
<tgroup cols="3">
<thead>
<row>
<entry>Function</entry>
<entry>Return Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal><function>pg_stat_get_db_numbackends</function>(<type>oid</type>)</literal></entry>
<entry><type>integer</type></entry>
<entry>
Number of active server processes for database
</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_db_xact_commit</function>(<type>oid</type>)</literal></entry>
<entry><type>bigint</type></entry>
<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_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
</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_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_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 (null if the
current user is not a superuser nor the same user as that of
the session being queried, or
<varname>stats_command_string</varname> is not 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 (null if the
current user is not a superuser nor the same user as that of
the session being queried, or
<varname>stats_command_string</varname> is not 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_reset</function>()</literal></entry>
<entry><type>boolean</type></entry>
<entry>
Reset all currently collected statistics
</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>
</chapter>
<!-- Keep this comment at the end of the file
Local variables:
mode:sgml
sgml-omittag:nil
sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
sgml-parent-document:nil
sgml-default-dtd-file:"./reference.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:("/usr/lib/sgml/catalog")
sgml-local-ecat-files:nil
End:
-->
View Git Blame Copy Permalink