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postgres/doc/src/sgml/monitoring.sgml
Peter Eisentraut bc49968764 Add more appropriate markup.
2002-09-21 18:32:54 +00:00

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<chapter id="monitoring">
<title>Monitoring Database Activity</title>
<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 <firstterm>statistics collector</>,
but one should not neglect regular Unix monitoring programs such as
<command>ps</> and <command>top</>. Also, once one has identified a
poorly-performing query, further investigation may be needed using
<productname>PostgreSQL</productname>'s <command>EXPLAIN</> command.
The <citetitle>User's Guide</citetitle> 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
<firstterm>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</> (ie, waiting for a client command),
<literal>idle in transaction</> (waiting for client inside a BEGIN block),
or a command type name such as <literal>SELECT</>. Also,
<literal>waiting</> is attached if the server 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 <command>w</command>
flags, not just one. In addition, your original invocation of the
<application>postmaster</application> must have a shorter
<command>ps</command> status display than that provided by each
backend. If you fail to do all three things, the <command>ps</>
output for each backend will be the original <application>postmaster</>
command line.
</para>
</tip>
</sect1>
<sect1 id="monitoring-stats">
<title>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 query 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 variables that are normally set in
<filename>postgresql.conf</> (see <xref linkend="runtime-config"> for
details about setting configuration variables).
</para>
<para>
The variable <varname>STATS_START_COLLECTOR</varname> must be set to
<literal>true</> for the statistics collector to
be launched at all. This is the default and recommended setting,
but it may be turned off if you have no interest in statistics and
want to squeeze out every last drop of overhead. (The savings is
likely to be small, however.) Note that this option
cannot be changed while the server is running.
</para>
<para>
The variables <varname>STATS_COMMAND_STRING</varname>,
<varname>STATS_BLOCK_LEVEL</varname>,
and <varname>STATS_ROW_LEVEL</varname> control how much information is
actually sent to the collector, and thus determine how much run-time
overhead occurs. These respectively determine whether a server process
sends its current command string, disk-block-level access statistics, and
row-level access statistics to the collector. Normally these variables 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 server
processes using the <command>SET</> command. (To prevent ordinary users
from hiding their activity from the administrator, only superusers are
allowed to change these variables with <command>SET</>.)
</para>
<important>
<para>
Since the variables <varname>STATS_COMMAND_STRING</varname>,
<varname>STATS_BLOCK_LEVEL</varname>,
and <varname>STATS_ROW_LEVEL</varname>
default to <literal>false</>, no statistics are actually collected
in the default configuration! You must turn one or more of them on
before you will get useful results from the statistical display
functions.
</para>
</important>
</sect2>
<sect2 id="monitoring-stats-views">
<title>Viewing Collected Statistics</Title>
<para>
Several predefined views 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 access counts to the collector
just before waiting for another client command; so a query still in
progress does not affect the displayed totals. Also, the collector itself
emits new totals at most once per <varname>pgstat_stat_interval</varname> (500 milliseconds
by default). So the displayed totals lag behind actual activity.
</para>
<para>
Another important point is that when a server process is asked to display
any of these statistics, it first fetches the most recent totals emitted by
the collector process. It 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>
<title>Standard Statistics Views</title>
<tgroup cols="2">
<thead>
<row>
<entry>View Name</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structname>pg_stat_activity</></entry>
<entry>One row per server process, showing process <acronym>PID</>, database,
user, and current query. The current query column is only available
to superusers; for others it reads as NULL. (Note that because of
the collector's reporting delay, current query will only be up-to-date
for long-running queries.)</entry>
</row>
<row>
<entry><structname>pg_stat_database</></entry>
<entry>One row per database, showing number of active backends,
total transactions committed and total rolled back in that database,
total disk blocks read, and total number of buffer hits (ie, block
read requests avoided by finding the block already in buffer cache).
</entry>
</row>
<row>
<entry><structname>pg_stat_all_tables</></entry>
<entry>For each table in the current database, total numbers of
sequential and index scans, total numbers of tuples returned by
each type of scan, and totals of tuple insertions, updates,
and deletes.</entry>
</row>
<row>
<entry><structname>pg_stat_sys_tables</></entry>
<entry>Same as pg_stat_all_tables, except that only system tables
are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_user_tables</></entry>
<entry>Same as pg_stat_all_tables, except that only user tables
are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_all_indexes</></entry>
<entry>For each index in the current database, the total number
of index scans that have used that index, the number of index tuples
read, and the number of successfully fetched heap tuples (this may
be less when there are index entries pointing to expired heap tuples).
</entry>
</row>
<row>
<entry><structname>pg_stat_sys_indexes</></entry>
<entry>Same as 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 pg_stat_all_indexes, except that only indexes on
user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_tables</></entry>
<entry>For each table in the current database, the total number of disk
blocks read from that table, the number of buffer hits, the numbers of
disk blocks read and buffer hits in all the indexes of that table,
the numbers of disk blocks read and buffer hits from the table's
auxiliary TOAST table (if any), and the numbers of disk blocks read
and buffer hits for the TOAST table's index.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_tables</></entry>
<entry>Same as <structname>pg_statio_all_tables</>, except that only system tables
are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_user_tables</></entry>
<entry>Same as <structname>pg_statio_all_tables</>, except that only user tables
are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_indexes</></entry>
<entry>For each index in the current database, the numbers of
disk blocks read and buffer hits in that index.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_indexes</></entry>
<entry>Same as <structname>pg_statio_all_indexes</>, except that only indexes on
system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_user_indexes</></entry>
<entry>Same as <structname>pg_statio_all_indexes</>, except that only indexes on
user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_sequences</></entry>
<entry>For each sequence object in the current database, the numbers
of disk blocks read and buffer hits in that sequence.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_sequences</></entry>
<entry>Same as <structname>pg_statio_all_sequences</>, except that only system
sequences are shown. (Presently, no system sequences are defined,
so this view is always empty.)</entry>
</row>
<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>
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.
</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. The per-database access functions accept a database OID to
identify which database to report on. The per-table and per-index
functions accept a table or index OID (note that only tables and indexes
in the current
database can be seen with these functions). The per-backend access
functions accept a backend ID number, which ranges from one to the number
of currently active backends.
</para>
<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><function>pg_stat_get_db_numbackends</function>(<type>oid</type>)</entry>
<entry><type>integer</type></entry>
<entry>
Number of active backends in database
</entry>
</row>
<row>
<entry><function>pg_stat_get_db_xact_commit</function>(<type>oid</type>)</entry>
<entry><type>bigint</type></entry>
<entry>
Transactions committed in database
</entry>
</row>
<row>
<entry><function>pg_stat_get_db_xact_rollback</function>(<type>oid</type>)</entry>
<entry><type>bigint</type></entry>
<entry>
Transactions rolled back in database
</entry>
</row>
<row>
<entry><function>pg_stat_get_db_blocks_fetched</function>(<type>oid</type>)</entry>
<entry><type>bigint</type></entry>
<entry>
Number of disk block fetch requests for database
</entry>
</row>
<row>
<entry><function>pg_stat_get_db_blocks_hit</function>(<type>oid</type>)</entry>
<entry><type>bigint</type></entry>
<entry>
Number of disk block requests found in cache for database
</entry>
</row>
<row>
<entry><function>pg_stat_get_numscans</function>(<type>oid</type>)</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><function>pg_stat_get_tuples_returned</function>(<type>oid</type>)</entry>
<entry><type>bigint</type></entry>
<entry>
Number of tuples read by sequential scans when argument is a table,
or number of index tuples read when argument is an index
</entry>
</row>
<row>
<entry><function>pg_stat_get_tuples_fetched</function>(<type>oid</type>)</entry>
<entry><type>bigint</type></entry>
<entry>
Number of valid (unexpired) table tuples fetched by sequential scans
when argument is a table, or fetched by index scans using this index
when argument is an index
</entry>
</row>
<row>
<entry><function>pg_stat_get_tuples_inserted</function>(<type>oid</type>)</entry>
<entry><type>bigint</type></entry>
<entry>
Number of tuples inserted into table
</entry>
</row>
<row>
<entry><function>pg_stat_get_tuples_updated</function>(<type>oid</type>)</entry>
<entry><type>bigint</type></entry>
<entry>
Number of tuples updated in table
</entry>
</row>
<row>
<entry><function>pg_stat_get_tuples_deleted</function>(<type>oid</type>)</entry>
<entry><type>bigint</type></entry>
<entry>
Number of tuples deleted from table
</entry>
</row>
<row>
<entry><function>pg_stat_get_blocks_fetched</function>(<type>oid</type>)</entry>
<entry><type>bigint</type></entry>
<entry>
Number of disk block fetch requests for table or index
</entry>
</row>
<row>
<entry><function>pg_stat_get_blocks_hit</function>(<type>oid</type>)</entry>
<entry><type>bigint</type></entry>
<entry>
Number of disk block requests found in cache for table or index
</entry>
</row>
<row>
<entry><function>pg_stat_get_backend_idset</function>()</entry>
<entry><type>set of integer</type></entry>
<entry>
Set of currently active backend IDs (from 1 to N where N is the
number of active backends). See usage example below.
</entry>
</row>
<row>
<entry><function>pg_backend_pid</function>()</entry>
<entry><type>integer</type></entry>
<entry>
Process ID of the attached backend
</entry>
</row>
<row>
<entry><function>pg_stat_get_backend_pid</function>(<type>integer</type>)</entry>
<entry><type>integer</type></entry>
<entry>
Process ID of all backend processes
</entry>
</row>
<row>
<entry><function>pg_stat_get_backend_dbid</function>(<type>integer</type>)</entry>
<entry><type>oid</type></entry>
<entry>
Database ID of backend process
</entry>
</row>
<row>
<entry><function>pg_stat_get_backend_userid</function>(<type>integer</type>)</entry>
<entry><type>oid</type></entry>
<entry>
User ID of backend process
</entry>
</row>
<row>
<entry><function>pg_stat_get_backend_activity</function>(<type>integer</type>)</entry>
<entry><type>text</type></entry>
<entry>
Current query of backend process (NULL if caller is not superuser)
</entry>
</row>
<row>
<entry><function>pg_stat_reset</function>()</entry>
<entry><type>boolean</type></entry>
<entry>
Reset all currently collected statistics.
</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
Note: blocks_fetched minus blocks_hit gives the number of kernel 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>
<para>
The function <function>pg_stat_get_backend_idset</function> provides
a convenient way to generate one row for each active backend. For
example, to show the <acronym>PID</>s and current queries of all backends:
<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>
<para>
Another useful tool for monitoring database activity is the
<literal>pg_locks</literal> system catalog. This 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> backend.
</para>
</listitem>
<listitem>
<para>
View the relation in the current database with the most
un-granted 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>
For more information on locking and managing concurrency with
<productname>PostgreSQL</productname>, refer to the
<citetitle>Administrator's Guide</citetitle>.
</para>
<note>
<para>
When the <literal>pg_locks</literal> view is accessed, the
internal lock manager data structures are momentarily locked,
and a copy is made for the view to display. This ensures that
the view produces a consistent set of results, while not blocking
normal lock manager operations longer than necessary. Nonetheless
there could be some impact on database performance if this view is
examined often.
</para>
</note>
<para>
The <literal>pg_locks</literal> view contains one row per lockable
object and requested lock mode. Thus, the same lockable object
may appear many times, if multiple transactions are holding or
waiting for locks on it. A lockable object is either a relation
or a transaction ID. (Note that this view includes only table-level
locks, not row-level ones. If a transaction is waiting for a
row-level lock, it will appear in the view as waiting for the
transaction ID of the current holder of that row lock.)
</para>
<table>
<title>Lock Status System View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column Name</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>relation</structfield></entry>
<entry><type>oid</type></entry>
<entry>The OID of the locked relation, or NULL if the lockable
object is a transaction ID. This column can be joined
with the <literal>pg_class</literal> system catalog to get more
information on the locked relation. Note however that this will
only work for relations in the current database (those for which
the <structfield>database</structfield> column is either the
current database's OID or zero).
</entry>
</row>
<row>
<entry><structfield>database</structfield></entry>
<entry><type>oid</type></entry>
<entry>The OID of the database in which the locked relation
exists, or NULL if the lockable object is a transaction ID.
If the lock is on a globally-shared table, this field will be
zero. This
column can be joined with the <literal>pg_database</literal>
system catalog to get more information on the locked object's
database.
</entry>
</row>
<row>
<entry><structfield>transaction</structfield></entry>
<entry><type>xid</type></entry>
<entry>The ID of a transaction, or NULL if the lockable object
is a relation. Every transaction holds an exclusive lock on its
transaction ID for its entire duration. If one transaction finds
it necessary to wait specifically for another transaction, it
does so by attempting to acquire share lock on the other transaction
ID. That will succeed only when the other transaction terminates
and releases its locks.
</entry>
</row>
<row>
<entry><structfield>pid</structfield></entry>
<entry><type>int4</type></entry>
<entry>The process ID of the
<productname>PostgreSQL</productname> backend that has
acquired or is attempting to acquire the lock. If you have
enabled the statistics collector, this column can be joined
with the <literal>pg_stat_activity</literal> view to get
more information on the backend holding or waiting to hold the
lock.</entry>
</row>
<row>
<entry><structfield>mode</structfield></entry>
<entry><type>text</type></entry>
<entry>The mode of the requested or held lock on the lockable
object. For more information on the
different lock modes available in
<productname>PostgreSQL</productname>, refer to the
<citetitle>User's Guide</citetitle>.</entry>
</row>
<row>
<entry><structfield>isgranted</structfield></entry>
<entry><type>bool</type></entry>
<entry>True if this lock has been granted (is held by this
backend). False indicates that this backend is currently
waiting to acquire this lock, which implies that some other
backend is holding a conflicting lock mode on the same lockable
object. This backend will sleep until the other lock is released
(or a deadlock situation is detected). A single backend can be
waiting to acquire at most one lock at a time.</entry>
</row>
</tbody>
</tgroup>
</table>
</sect1>
</chapter>
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