<!-- doc/src/sgml/monitoring.sgml -->
<chapter id="monitoring">
<title>Monitoring Database Activity</title>
<indexterm zone="monitoring">
<primary>monitoring</primary>
<secondary>database activity</secondary>
</indexterm>
<indexterm zone="monitoring">
<primary>database activity</primary>
<secondary>monitoring</secondary>
</indexterm>
<para>
A database administrator frequently wonders, <quote>What is the system
doing right now?</quote>
This chapter discusses how to find that out.
</para>
<para>
Several tools are available for monitoring database activity and
analyzing performance. Most of this chapter is devoted to describing
<productname>PostgreSQL</productname>'s statistics collector,
but one should not neglect regular Unix monitoring programs such as
<command>ps</>, <command>top</>, <command>iostat</>, and <command>vmstat</>.
Also, once one has identified a
poorly-performing query, further investigation might be needed using
<productname>PostgreSQL</productname>'s <xref linkend="sql-explain"> command.
<xref linkend="using-explain"> discusses <command>EXPLAIN</>
and other methods for understanding the behavior of an individual
query.
</para>
<sect1 id="monitoring-ps">
<title>Standard Unix Tools</title>
<indexterm zone="monitoring-ps">
<primary>ps</primary>
<secondary>to monitor activity</secondary>
</indexterm>
<para>
On most Unix platforms, <productname>PostgreSQL</productname> modifies its
command title as reported by <command>ps</>, so that individual server
processes can readily be identified. A sample display is
<screen>
$ ps auxww | grep ^gaussdb
gaussdb 15551 0.0 0.1 57536 7132 pts/0 S 18:02 0:00 gaussdb -i
gaussdb 15554 0.0 0.0 57536 1184 ? Ss 18:02 0:00 gaussdb: writer process
gaussdb 15555 0.0 0.0 57536 916 ? Ss 18:02 0:00 gaussdb: checkpointer process
gaussdb 15556 0.0 0.0 57536 916 ? Ss 18:02 0:00 gaussdb: wal writer process
gaussdb 15557 0.0 0.0 58504 2244 ? Ss 18:02 0:00 gaussdb: autovacuum launcher process
gaussdb 15558 0.0 0.0 17512 1068 ? Ss 18:02 0:00 gaussdb: stats collector process
gaussdb 15582 0.0 0.0 58772 3080 ? Ss 18:04 0:00 gaussdb: joe runbug 127.0.0.1 idle
gaussdb 15606 0.0 0.0 58772 3052 ? Ss 18:07 0:00 gaussdb: tgl regression [local] SELECT waiting
gaussdb 15610 0.0 0.0 58772 3056 ? Ss 18:07 0:00 gaussdb: tgl regression [local] idle in transaction
</screen>
(The appropriate invocation of <command>ps</> varies across different
platforms, as do the details of what is shown. This example is from a
recent Linux system.) The first process listed here is the
master server process. The command arguments
shown for it are the same ones used when it was launched. The next five
processes are background worker processes automatically launched by the
master process. (The <quote>stats collector</> process will not be present
if you have set the system not to start the statistics collector; likewise
the <quote>autovacuum launcher</> process can be disabled.)
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>
gaussdb: <replaceable>user</> <replaceable>database</> <replaceable>host</> <replaceable>activity</>
</screen>
The user, database, and (client) host items remain the same for
the life of the client connection, but the activity indicator changes.
The activity can be <literal>idle</> (i.e., waiting for a client command),
<literal>idle in transaction</> (waiting for client inside a <command>BEGIN</> block),
or a command type name such as <literal>SELECT</>. Also,
<literal>waiting</> is appended if the server process is presently waiting
on a lock held by another session. In the above example we can infer
that process 15606 is waiting for process 15610 to complete its transaction
and thereby release some lock. (Process 15610 must be the blocker, because
there is no other active session. In more complicated cases it would be
necessary to look into the
<link linkend="view-pg-locks"><structname>pg_locks</structname></link>
system view to determine who is blocking whom.)
</para>
<para>
If you have turned off <xref linkend="guc-update-process-title"> then the
activity indicator is not updated; the process title is set only once
when a new process is launched. On some platforms this saves a measurable
amount of per-command overhead; on others it's insignificant.
</para>
<tip>
<para>
<productname>Solaris</productname> requires special handling. You must
use <command>/usr/ucb/ps</command>, rather than
<command>/bin/ps</command>. You also must use two <option>w</option>
flags, not just one. In addition, your original invocation of the
<command>gaussdb</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>gaussdb</>
command line.
</para>
</tip>
</sect1>
<sect1 id="monitoring-stats">
<title>The Statistics Collector</title>
<indexterm zone="monitoring-stats">
<primary>statistics</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname>'s <firstterm>statistics collector</>
is a subsystem that supports collection and reporting of information about
server activity. Presently, the collector can count accesses to tables
and indexes in both disk-block and individual-row terms. It also tracks
the total number of rows in each table, and information about vacuum and
analyze actions for each table. It can also count calls to user-defined
functions and the total time spent in each one.
</para>
<para>
<productname>PostgreSQL</productname> also supports reporting of the exact
command currently being executed by other server processes. This
facility is independent of the collector process.
</para>
<sect2 id="monitoring-stats-setup">
<title>Statistics Collection Configuration</title>
<para>
Since collection of statistics adds some overhead to query execution,
the system can be configured to collect or not collect information.
This is controlled by configuration parameters that are normally set in
<filename>postgresql.conf</>. (See <xref linkend="runtime-config"> for
details about setting configuration parameters.)
</para>
<para>
The parameter <xref linkend="guc-track-activities"> enables monitoring
of the current command being executed by any server process.
</para>
<para>
The parameter <xref linkend="guc-track-counts"> controls whether
statistics are collected about table and index accesses.
</para>
<para>
The parameter <xref linkend="guc-track-functions"> enables tracking of
usage of user-defined functions.
</para>
<para>
The parameter <xref linkend="guc-track-io-timing"> enables monitoring
of block read and write times.
</para>
<para>
Normally these parameters are set in <filename>postgresql.conf</> so
that they apply to all server processes, but it is possible to turn
them on or off in individual sessions using the <xref
linkend="sql-set"> command. (To prevent
ordinary users from hiding their activity from the administrator,
only superusers are allowed to change these parameters with
<command>SET</>.)
</para>
<para>
The statistics collector transmits the collected information to other
<productname>PostgreSQL</productname> processes through temporary files.
These files are stored in the directory named by the
<xref linkend="guc-stats-temp-directory"> parameter,
<filename>pg_stat_tmp</filename> by default.
For better performance, <varname>stats_temp_directory</> can be
pointed at a RAM-based file system, decreasing physical I/O requirements.
When the server shuts down, a permanent copy of the statistics
data is stored in the <filename>global</filename> subdirectory, so that
statistics can be retained across server restarts.
</para>
</sect2>
<sect2 id="monitoring-stats-views">
<title>Viewing Collected Statistics</title>
<para>
Several predefined views, listed in <xref
linkend="monitoring-stats-views-table">, are available to show the results
of statistics collection. Alternatively, one can
build custom views using the underlying statistics functions, as discussed
in <xref linkend="monitoring-stats-functions">.
</para>
<para>
When using the statistics to monitor current activity, it is important
to realize that the information does not update instantaneously.
Each individual server process transmits new statistical counts to
the collector just before going idle; so a query or transaction still in
progress does not affect the displayed totals. Also, the collector itself
emits a new report at most once per <varname>PGSTAT_STAT_INTERVAL</varname>
milliseconds (500 ms unless altered while building the server). So the
displayed information lags behind actual activity. However, current-query
information collected by <varname>track_activities</varname> is
always up-to-date.
</para>
<para>
Another important point is that when a server process is asked to display
any of these statistics, it first fetches the most recent report emitted by
the collector process and then continues to use this snapshot for all
statistical views and functions until the end of its current transaction.
So the statistics will show static information as long as you continue the
current transaction. Similarly, information about the current queries of
all sessions is collected when any such information is first requested
within a transaction, and the same information will be displayed throughout
the transaction.
This is a feature, not a bug, because it allows you to perform several
queries on the statistics and correlate the results without worrying that
the numbers are changing underneath you. But if you want to see new
results with each query, be sure to do the queries outside any transaction
block. Alternatively, you can invoke
<function>pg_stat_clear_snapshot</function>(), which will discard the
current transaction's statistics snapshot (if any). The next use of
statistical information will cause a new snapshot to be fetched.
</para>
<para>
A transaction can also see its own statistics (as yet untransmitted to the
collector) in the views <structname>pg_stat_xact_all_tables</>,
<structname>pg_stat_xact_sys_tables</>,
<structname>pg_stat_xact_user_tables</>, and
<structname>pg_stat_xact_user_functions</>. These numbers do not act as
stated above; instead they update continuously throughout the transaction.
</para>
<table id="monitoring-stats-views-table">
<title>Standard Statistics Views</title>
<tgroup cols="2">
<thead>
<row>
<entry>View Name</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>
<structname>pg_stat_activity</structname>
<indexterm><primary>pg_stat_activity</primary></indexterm>
</entry>
<entry>
One row per server process, showing information related to
the current activity of that process, such as state and current query.
See <xref linkend="pg-stat-activity-view"> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_bgwriter</><indexterm><primary>pg_stat_bgwriter</primary></indexterm></entry>
<entry>One row only, showing statistics about the
background writer process's activity. See
<xref linkend="pg-stat-bgwriter-view"> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_database</><indexterm><primary>pg_stat_database</primary></indexterm></entry>
<entry>One row per database, showing database-wide statistics. See
<xref linkend="pg-stat-database-view"> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_all_tables</><indexterm><primary>pg_stat_all_tables</primary></indexterm></entry>
<entry>
One row for each table in the current database, showing statistics
about accesses to that specific table.
See <xref linkend="pg-stat-all-tables-view"> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_sys_tables</><indexterm><primary>pg_stat_sys_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_all_tables</>, except that only
system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_user_tables</><indexterm><primary>pg_stat_user_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_all_tables</>, except that only user
tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_xact_all_tables</><indexterm><primary>pg_stat_xact_all_tables</primary></indexterm></entry>
<entry>Similar to <structname>pg_stat_all_tables</>, but counts actions
taken so far within the current transaction (which are <emphasis>not</>
yet included in <structname>pg_stat_all_tables</> and related views).
The columns for numbers of live and dead rows and vacuum and
analyze actions are not present in this view.</entry>
</row>
<row>
<entry><structname>pg_stat_xact_sys_tables</><indexterm><primary>pg_stat_xact_sys_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_xact_all_tables</>, except that only
system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_xact_user_tables</><indexterm><primary>pg_stat_xact_user_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_xact_all_tables</>, except that only
user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_all_indexes</><indexterm><primary>pg_stat_all_indexes</primary></indexterm></entry>
<entry>
One row for each index in the current database, showing statistics
about accesses to that specific index.
See <xref linkend="pg-stat-all-indexes-view"> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_sys_indexes</><indexterm><primary>pg_stat_sys_indexes</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_all_indexes</>, except that only
indexes on system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_user_indexes</><indexterm><primary>pg_stat_user_indexes</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_all_indexes</>, except that only
indexes on user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_tables</><indexterm><primary>pg_statio_all_tables</primary></indexterm></entry>
<entry>
One row for each table in the current database, showing statistics
about I/O on that specific table.
See <xref linkend="pg-statio-all-tables-view"> for details.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_tables</><indexterm><primary>pg_statio_sys_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_tables</>, except that only
system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_user_tables</><indexterm><primary>pg_statio_user_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_tables</>, except that only
user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_indexes</><indexterm><primary>pg_statio_all_indexes</primary></indexterm></entry>
<entry>
One row for each index in the current database,
showing statistics about I/O on that specific index.
See <xref linkend="pg-statio-all-indexes-view"> for details.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_indexes</><indexterm><primary>pg_statio_sys_indexes</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_indexes</>, except that only
indexes on system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_user_indexes</><indexterm><primary>pg_statio_user_indexes</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_indexes</>, except that only
indexes on user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_sequences</><indexterm><primary>pg_statio_all_sequences</primary></indexterm></entry>
<entry>
One row for each sequence in the current database,
showing statistics about I/O on that specific sequence.
See <xref linkend="pg-statio-all-sequences-view"> for details.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_sequences</><indexterm><primary>pg_statio_sys_sequences</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_sequences</>, except that only
system sequences are shown. (Presently, no system sequences are defined,
so this view is always empty.)</entry>
</row>
<row>
<entry><structname>pg_statio_user_sequences</><indexterm><primary>pg_statio_user_sequences</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_sequences</>, except that only
user sequences are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_user_functions</><indexterm><primary>pg_stat_user_functions</primary></indexterm></entry>
<entry>
One row for each tracked function, showing statistics
about executions of that function. See
<xref linkend="pg-stat-user-functions-view"> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_xact_user_functions</><indexterm><primary>pg_stat_xact_user_functions</primary></indexterm></entry>
<entry>Similar to <structname>pg_stat_user_functions</>, but counts only
calls during the current transaction (which are <emphasis>not</>
yet included in <structname>pg_stat_user_functions</>).</entry>
</row>
<row>
<entry><structname>pg_stat_replication</><indexterm><primary>pg_stat_replication</primary></indexterm></entry>
<entry>One row per WAL sender process, showing statistics about
replication to that sender's connected standby server.
See <xref linkend="pg-stat-replication-view"> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_database_conflicts</><indexterm><primary>pg_stat_database_conflicts</primary></indexterm></entry>
<entry>
One row per database, showing database-wide statistics about
query cancels due to conflict with recovery on standby servers.
See <xref linkend="pg-stat-database-conflicts-view"> for details.
</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. However, these statistics do not give the
entire story: due to the way in which <productname>PostgreSQL</>
handles disk I/O, data that is not in the
<productname>PostgreSQL</> buffer cache might still reside in the
kernel's I/O cache, and might therefore still be fetched without
requiring a physical read. Users interested in obtaining more
detailed information on <productname>PostgreSQL</> I/O behavior are
advised to use the <productname>PostgreSQL</> statistics collector
in combination with operating system utilities that allow insight
into the kernel's handling of I/O.
</para>
<table id="pg-stat-activity-view" xreflabel="pg_stat_activity">
<title><structname>pg_stat_activity</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>datid</></entry>
<entry><type>oid</></entry>
<entry>OID of the database this backend is connected to</entry>
</row>
<row>
<entry><structfield>datname</></entry>
<entry><type>name</></entry>
<entry>Name of the database this backend is connected to</entry>
</row>
<row>
<entry><structfield>pid</></entry>
<entry><type>integer</></entry>
<entry>Process ID of this backend</entry>
</row>
<row>
<entry><structfield>usesysid</></entry>
<entry><type>oid</></entry>
<entry>OID of the user logged into this backend</entry>
</row>
<row>
<entry><structfield>usename</></entry>
<entry><type>name</></entry>
<entry>Name of the user logged into this backend</entry>
</row>
<row>
<entry><structfield>application_name</></entry>
<entry><type>text</></entry>
<entry>Name of the application that is connected
to this backend</entry>
</row>
<row>
<entry><structfield>client_addr</></entry>
<entry><type>inet</></entry>
<entry>IP address of the client connected to this backend.
If this field is null, it indicates either that the client is
connected via a Unix socket on the server machine or that this is an
internal process such as autovacuum.
</entry>
</row>
<row>
<entry><structfield>client_hostname</></entry>
<entry><type>text</></entry>
<entry>Host name of the connected client, as reported by a
reverse DNS lookup of <structfield>client_addr</>. This field will
only be non-null for IP connections, and only when <xref
linkend="guc-log-hostname"> is enabled.
</entry>
</row>
<row>
<entry><structfield>client_port</></entry>
<entry><type>integer</></entry>
<entry>TCP port number that the client is using for communication
with this backend, or <literal>-1</> if a Unix socket is used
</entry>
</row>
<row>
<entry><structfield>backend_start</></entry>
<entry><type>timestamp with time zone</></entry>
<entry>Time when this process was started, i.e., when the
client connected to the server
</entry>
</row>
<row>
<entry><structfield>xact_start</></entry>
<entry><type>timestamp with time zone</></entry>
<entry>Time when this process' current transaction was started, or null
if no transaction is active. If the current
query is the first of its transaction, this column is equal to the
<structfield>query_start</> column.
</entry>
</row>
<row>
<entry><structfield>query_start</></entry>
<entry><type>timestamp with time zone</></entry>
<entry>Time when the currently active query was started, or if
<structfield>state</> is not <literal>active</>, when the last query
was started
</entry>
</row>
<row>
<entry><structfield>state_change</></entry>
<entry><type>timestamp with time zone</></entry>
<entry>Time when the <structfield>state</> was last changed</entry>
</row>
<row>
<entry><structfield>waiting</></entry>
<entry><type>boolean</></entry>
<entry>True if this backend is currently waiting on a lock</entry>
</row>
<row>
<entry><structfield>state</></entry>
<entry><type>text</></entry>
<entry>Current overall state of this backend.
Possible values are:
<itemizedlist>
<listitem>
<para>
<literal>active</>: The backend is executing a query.
</para>
</listitem>
<listitem>
<para>
<literal>idle</>: The backend is waiting for a new client command.
</para>
</listitem>
<listitem>
<para>
<literal>idle in transaction</>: The backend is in a transaction,
but is not currently executing a query.
</para>
</listitem>
<listitem>
<para>
<literal>idle in transaction (aborted)</>: This state is similar to
<literal>idle in transaction</>, except one of the statements in
the transaction caused an error.
</para>
</listitem>
<listitem>
<para>
<literal>fastpath function call</>: The backend is executing a
fast-path function.
</para>
</listitem>
<listitem>
<para>
<literal>disabled</>: This state is reported if <xref
linkend="guc-track-activities"> is disabled in this backend.
</para>
</listitem>
</itemizedlist>
</entry>
</row>
<row>
<entry><structfield>query</></entry>
<entry><type>text</></entry>
<entry>Text of this backend's most recent query. If
<structfield>state</> is <literal>active</> this field shows the
currently executing query. In all other states, it shows the last query
that was executed.
</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_activity</structname> view will have one row
per server process, showing information related to
the current activity of that process.
</para>
<note>
<para>
The <structfield>waiting</> and <structfield>state</> columns are
independent. If a backend is in the <literal>active</> state,
it may or may not be <literal>waiting</>. If the state is
<literal>active</> and <structfield>waiting</> is true, it means
that a query is being executed, but is being blocked by a lock
somewhere in the system.
</para>
</note>
<table id="pg-stat-bgwriter-view" xreflabel="pg_stat_bgwriter">
<title><structname>pg_stat_bgwriter</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>checkpoints_timed</></entry>
<entry><type>bigint</type></entry>
<entry>Number of scheduled checkpoints that have been performed</entry>
</row>
<row>
<entry><structfield>checkpoints_req</></entry>
<entry><type>bigint</type></entry>
<entry>Number of requested checkpoints that have been performed</entry>
</row>
<row>
<entry><structfield>checkpoint_write_time</></entry>
<entry><type>double precision</type></entry>
<entry>
Total amount of time that has been spent in the portion of
checkpoint processing where files are written to disk, in milliseconds
</entry>
</row>
<row>
<entry><structfield>checkpoint_sync_time</></entry>
<entry><type>double precision</type></entry>
<entry>
Total amount of time that has been spent in the portion of
checkpoint processing where files are synchronized to disk, in
milliseconds
</entry>
</row>
<row>
<entry><structfield>buffers_checkpoint</></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffers written during checkpoints</entry>
</row>
<row>
<entry><structfield>buffers_clean</></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffers written by the background writer</entry>
</row>
<row>
<entry><structfield>maxwritten_clean</></entry>
<entry><type>bigint</type></entry>
<entry>Number of times the background writer stopped a cleaning
scan because it had written too many buffers</entry>
</row>
<row>
<entry><structfield>buffers_backend</></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffers written directly by a backend</entry>
</row>
<row>
<entry><structfield>buffers_backend_fsync</></entry>
<entry><type>bigint</type></entry>
<entry>Number of times a backend had to execute its own
<function>fsync</> call (normally the background writer handles those
even when the backend does its own write)</entry>
</row>
<row>
<entry><structfield>buffers_alloc</></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffers allocated</entry>
</row>
<row>
<entry><structfield>stats_reset</></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time at which these statistics were last reset</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_bgwriter</structname> view will always have a
single row, containing global data for the cluster.
</para>
<table id="pg-stat-database-view" xreflabel="pg_stat_database">
<title><structname>pg_stat_database</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>datid</></entry>
<entry><type>oid</></entry>
<entry>OID of a database</entry>
</row>
<row>
<entry><structfield>datname</></entry>
<entry><type>name</></entry>
<entry>Name of this database</entry>
</row>
<row>
<entry><structfield>numbackends</></entry>
<entry><type>integer</></entry>
<entry>Number of backends currently connected to this database.
This is the only column in this view that returns a value reflecting
current state; all other columns return the accumulated values since
the last reset.</entry>
</row>
<row>
<entry><structfield>xact_commit</></entry>
<entry><type>bigint</></entry>
<entry>Number of transactions in this database that have been
committed</entry>
</row>
<row>
<entry><structfield>xact_rollback</></entry>
<entry><type>bigint</></entry>
<entry>Number of transactions in this database that have been
rolled back</entry>
</row>
<row>
<entry><structfield>blks_read</></entry>
<entry><type>bigint</></entry>
<entry>Number of disk blocks read in this database</entry>
</row>
<row>
<entry><structfield>blks_hit</></entry>
<entry><type>bigint</></entry>
<entry>Number of times disk blocks were found already in the buffer
cache, so that a read was not necessary (this only includes hits in the
PostgreSQL buffer cache, not the operating system's file system cache)
</entry>
</row>
<row>
<entry><structfield>tup_returned</></entry>
<entry><type>bigint</></entry>
<entry>Number of rows returned by queries in this database</entry>
</row>
<row>
<entry><structfield>tup_fetched</></entry>
<entry><type>bigint</></entry>
<entry>Number of rows fetched by queries in this database</entry>
</row>
<row>
<entry><structfield>tup_inserted</></entry>
<entry><type>bigint</></entry>
<entry>Number of rows inserted by queries in this database</entry>
</row>
<row>
<entry><structfield>tup_updated</></entry>
<entry><type>bigint</></entry>
<entry>Number of rows updated by queries in this database</entry>
</row>
<row>
<entry><structfield>tup_deleted</></entry>
<entry><type>bigint</></entry>
<entry>Number of rows deleted by queries in this database</entry>
</row>
<row>
<entry><structfield>conflicts</></entry>
<entry><type>bigint</></entry>
<entry>Number of queries canceled due to conflicts with recovery
in this database. (Conflicts occur only on standby servers; see
<xref linkend="pg-stat-database-conflicts-view"> for details.)
</entry>
</row>
<row>
<entry><structfield>temp_files</></entry>
<entry><type>bigint</></entry>
<entry>Number of temporary files created by queries in this database.
All temporary files are counted, regardless of why the temporary file
was created (e.g., sorting or hashing), and regardless of the
<xref linkend="guc-log-temp-files"> setting.
</entry>
</row>
<row>
<entry><structfield>temp_bytes</></entry>
<entry><type>bigint</></entry>
<entry>Total amount of data written to temporary files by queries in
this database. All temporary files are counted, regardless of why
the temporary file was created, and
regardless of the <xref linkend="guc-log-temp-files"> setting.
</entry>
</row>
<row>
<entry><structfield>deadlocks</></entry>
<entry><type>bigint</></entry>
<entry>Number of deadlocks detected in this database</entry>
</row>
<row>
<entry><structfield>blk_read_time</></entry>
<entry><type>double precision</></entry>
<entry>Time spent reading data file blocks by backends in this database,
in milliseconds</entry>
</row>
<row>
<entry><structfield>blk_write_time</></entry>
<entry><type>double precision</></entry>
<entry>Time spent writing data file blocks by backends in this database,
in milliseconds</entry>
</row>
<row>
<entry><structfield>stats_reset</></entry>
<entry><type>timestamp with time zone</></entry>
<entry>Time at which these statistics were last reset</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_database</structname> view will contain one row
for each database in the cluster, showing database-wide statistics.
</para>
<table id="pg-stat-all-tables-view" xreflabel="pg_stat_all_tables">
<title><structname>pg_stat_all_tables</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>relid</></entry>
<entry><type>oid</></entry>
<entry>OID of a table</entry>
</row>
<row>
<entry><structfield>schemaname</></entry>
<entry><type>name</></entry>
<entry>Name of the schema that this table is in</entry>
</row>
<row>
<entry><structfield>relname</></entry>
<entry><type>name</></entry>
<entry>Name of this table</entry>
</row>
<row>
<entry><structfield>seq_scan</></entry>
<entry><type>bigint</></entry>
<entry>Number of sequential scans initiated on this table</entry>
</row>
<row>
<entry><structfield>seq_tup_read</></entry>
<entry><type>bigint</></entry>
<entry>Number of live rows fetched by sequential scans</entry>
</row>
<row>
<entry><structfield>idx_scan</></entry>
<entry><type>bigint</></entry>
<entry>Number of index scans initiated on this table</entry>
</row>
<row>
<entry><structfield>idx_tup_fetch</></entry>
<entry><type>bigint</></entry>
<entry>Number of live rows fetched by index scans</entry>
</row>
<row>
<entry><structfield>n_tup_ins</></entry>
<entry><type>bigint</></entry>
<entry>Number of rows inserted</entry>
</row>
<row>
<entry><structfield>n_tup_upd</></entry>
<entry><type>bigint</></entry>
<entry>Number of rows updated</entry>
</row>
<row>
<entry><structfield>n_tup_del</></entry>
<entry><type>bigint</></entry>
<entry>Number of rows deleted</entry>
</row>
<row>
<entry><structfield>n_tup_hot_upd</></entry>
<entry><type>bigint</></entry>
<entry>Number of rows HOT updated (i.e., with no separate index
update required)</entry>
</row>
<row>
<entry><structfield>n_live_tup</></entry>
<entry><type>bigint</></entry>
<entry>Estimated number of live rows</entry>
</row>
<row>
<entry><structfield>n_dead_tup</></entry>
<entry><type>bigint</></entry>
<entry>Estimated number of dead rows</entry>
</row>
<row>
<entry><structfield>last_vacuum</></entry>
<entry><type>timestamp with time zone</></entry>
<entry>Last time at which this table was manually vacuumed
(not counting <command>VACUUM FULL</>)</entry>
</row>
<row>
<entry><structfield>last_autovacuum</></entry>
<entry><type>timestamp with time zone</></entry>
<entry>Last time at which this table was vacuumed by the autovacuum
daemon</entry>
</row>
<row>
<entry><structfield>last_analyze</></entry>
<entry><type>timestamp with time zone</></entry>
<entry>Last time at which this table was manually analyzed</entry>
</row>
<row>
<entry><structfield>last_autoanalyze</></entry>
<entry><type>timestamp with time zone</></entry>
<entry>Last time at which this table was analyzed by the autovacuum
daemon</entry>
</row>
<row>
<entry><structfield>vacuum_count</></entry>
<entry><type>bigint</></entry>
<entry>Number of times this table has been manually vacuumed
(not counting <command>VACUUM FULL</>)</entry>
</row>
<row>
<entry><structfield>autovacuum_count</></entry>
<entry><type>bigint</></entry>
<entry>Number of times this table has been vacuumed by the autovacuum
daemon</entry>
</row>
<row>
<entry><structfield>analyze_count</></entry>
<entry><type>bigint</></entry>
<entry>Number of times this table has been manually analyzed</entry>
</row>
<row>
<entry><structfield>autoanalyze_count</></entry>
<entry><type>bigint</></entry>
<entry>Number of times this table has been analyzed by the autovacuum
daemon</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_all_tables</structname> view will contain
one row for each table in the current database (including TOAST
tables), showing statistics about accesses to that specific table. The
<structname>pg_stat_user_tables</structname> and
<structname>pg_stat_sys_tables</structname> views
contain the same information,
but filtered to only show user and system tables respectively.
</para>
<table id="pg-stat-all-indexes-view" xreflabel="pg_stat_all_indexes">
<title><structname>pg_stat_all_indexes</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>relid</></entry>
<entry><type>oid</></entry>
<entry>OID of the table for this index</entry>
</row>
<row>
<entry><structfield>indexrelid</></entry>
<entry><type>oid</></entry>
<entry>OID of this index</entry>
</row>
<row>
<entry><structfield>schemaname</></entry>
<entry><type>name</></entry>
<entry>Name of the schema this index is in</entry>
</row>
<row>
<entry><structfield>relname</></entry>
<entry><type>name</></entry>
<entry>Name of the table for this index</entry>
</row>
<row>
<entry><structfield>indexrelname</></entry>
<entry><type>name</></entry>
<entry>Name of this index</entry>
</row>
<row>
<entry><structfield>idx_scan</></entry>
<entry><type>bigint</></entry>
<entry>Number of index scans initiated on this index</entry>
</row>
<row>
<entry><structfield>idx_tup_read</></entry>
<entry><type>bigint</></entry>
<entry>Number of index entries returned by scans on this index</entry>
</row>
<row>
<entry><structfield>idx_tup_fetch</></entry>
<entry><type>bigint</></entry>
<entry>Number of live table rows fetched by simple index scans using this
index</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_all_indexes</structname> view will contain
one row for each index in the current database,
showing statistics about accesses to that specific index. The
<structname>pg_stat_user_indexes</structname> and
<structname>pg_stat_sys_indexes</structname> views
contain the same information,
but filtered to only show user and system indexes respectively.
</para>
<para>
Indexes can be used via either simple index scans or <quote>bitmap</>
index 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>
The <structfield>idx_tup_read</> and <structfield>idx_tup_fetch</> counts
can be different even without any use of bitmap scans,
because <structfield>idx_tup_read</> counts
index entries retrieved from the index while <structfield>idx_tup_fetch</>
counts live rows fetched from the table. The latter will be less if any
dead or not-yet-committed rows are fetched using the index, or if any
heap fetches are avoided by means of an index-only scan.
</para>
</note>
<table id="pg-statio-all-tables-view" xreflabel="pg_statio_all_tables">
<title><structname>pg_statio_all_tables</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>relid</></entry>
<entry><type>oid</></entry>
<entry>OID of a table</entry>
</row>
<row>
<entry><structfield>schemaname</></entry>
<entry><type>name</></entry>
<entry>Name of the schema that this table is in</entry>
</row>
<row>
<entry><structfield>relname</></entry>
<entry><type>name</></entry>
<entry>Name of this table</entry>
</row>
<row>
<entry><structfield>heap_blks_read</></entry>
<entry><type>bigint</></entry>
<entry>Number of disk blocks read from this table</entry>
</row>
<row>
<entry><structfield>heap_blks_hit</></entry>
<entry><type>bigint</></entry>
<entry>Number of buffer hits in this table</entry>
</row>
<row>
<entry><structfield>idx_blks_read</></entry>
<entry><type>bigint</></entry>
<entry>Number of disk blocks read from all indexes on this table</entry>
</row>
<row>
<entry><structfield>idx_blks_hit</></entry>
<entry><type>bigint</></entry>
<entry>Number of buffer hits in all indexes on this table</entry>
</row>
<row>
<entry><structfield>toast_blks_read</></entry>
<entry><type>bigint</></entry>
<entry>Number of disk blocks read from this table's TOAST table (if any)</entry>
</row>
<row>
<entry><structfield>toast_blks_hit</></entry>
<entry><type>bigint</></entry>
<entry>Number of buffer hits in this table's TOAST table (if any)</entry>
</row>
<row>
<entry><structfield>tidx_blks_read</></entry>
<entry><type>bigint</></entry>
<entry>Number of disk blocks read from this table's TOAST table index (if any)</entry>
</row>
<row>
<entry><structfield>tidx_blks_hit</></entry>
<entry><type>bigint</></entry>
<entry>Number of buffer hits in this table's TOAST table index (if any)</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_statio_all_tables</structname> view will contain
one row for each table in the current database (including TOAST
tables), showing statistics about I/O on that specific table. The
<structname>pg_statio_user_tables</structname> and
<structname>pg_statio_sys_tables</structname> views
contain the same information,
but filtered to only show user and system tables respectively.
</para>
<table id="pg-statio-all-indexes-view" xreflabel="pg_statio_all_indexes">
<title><structname>pg_statio_all_indexes</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>relid</></entry>
<entry><type>oid</></entry>
<entry>OID of the table for this index</entry>
</row>
<row>
<entry><structfield>indexrelid</></entry>
<entry><type>oid</></entry>
<entry>OID of this index</entry>
</row>
<row>
<entry><structfield>schemaname</></entry>
<entry><type>name</></entry>
<entry>Name of the schema this index is in</entry>
</row>
<row>
<entry><structfield>relname</></entry>
<entry><type>name</></entry>
<entry>Name of the table for this index</entry>
</row>
<row>
<entry><structfield>indexrelname</></entry>
<entry><type>name</></entry>
<entry>Name of this index</entry>
</row>
<row>
<entry><structfield>idx_blks_read</></entry>
<entry><type>bigint</></entry>
<entry>Number of disk blocks read from this index</entry>
</row>
<row>
<entry><structfield>idx_blks_hit</></entry>
<entry><type>bigint</></entry>
<entry>Number of buffer hits in this index</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_statio_all_indexes</structname> view will contain
one row for each index in the current database,
showing statistics about I/O on that specific index. The
<structname>pg_statio_user_indexes</structname> and
<structname>pg_statio_sys_indexes</structname> views
contain the same information,
but filtered to only show user and system indexes respectively.
</para>
<table id="pg-statio-all-sequences-view" xreflabel="pg_statio_all_sequences">
<title><structname>pg_statio_all_sequences</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>relid</></entry>
<entry><type>oid</></entry>
<entry>OID of a sequence</entry>
</row>
<row>
<entry><structfield>schemaname</></entry>
<entry><type>name</></entry>
<entry>Name of the schema this sequence is in</entry>
</row>
<row>
<entry><structfield>relname</></entry>
<entry><type>name</></entry>
<entry>Name of this sequence</entry>
</row>
<row>
<entry><structfield>blks_read</></entry>
<entry><type>bigint</></entry>
<entry>Number of disk blocks read from this sequence</entry>
</row>
<row>
<entry><structfield>blks_hit</></entry>
<entry><type>bigint</></entry>
<entry>Number of buffer hits in this sequence</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_statio_all_sequences</structname> view will contain
one row for each sequence in the current database,
showing statistics about I/O on that specific sequence.
</para>
<table id="pg-stat-user-functions-view" xreflabel="pg_stat_user_functions">
<title><structname>pg_stat_user_functions</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>funcid</></entry>
<entry><type>oid</></entry>
<entry>OID of a function</entry>
</row>
<row>
<entry><structfield>schemaname</></entry>
<entry><type>name</></entry>
<entry>Name of the schema this function is in</entry>
</row>
<row>
<entry><structfield>funcname</></entry>
<entry><type>name</></entry>
<entry>Name of this function</entry>
</row>
<row>
<entry><structfield>calls</></entry>
<entry><type>bigint</></entry>
<entry>Number of times this function has been called</entry>
</row>
<row>
<entry><structfield>total_time</></entry>
<entry><type>double precision</></entry>
<entry>Total time spent in this function and all other functions
called by it, in milliseconds</entry>
</row>
<row>
<entry><structfield>self_time</></entry>
<entry><type>double precision</></entry>
<entry>Total time spent in this function itself, not including
other functions called by it, in milliseconds</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_user_functions</structname> view will contain
one row for each tracked function, showing statistics about executions of
that function. The <xref linkend="guc-track-functions"> parameter
controls exactly which functions are tracked.
</para>
<table id="pg-stat-replication-view" xreflabel="pg_stat_replication">
<title><structname>pg_stat_replication</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>pid</></entry>
<entry><type>integer</></entry>
<entry>Process ID of a WAL sender process</entry>
</row>
<row>
<entry><structfield>usesysid</></entry>
<entry><type>oid</></entry>
<entry>OID of the user logged into this WAL sender process</entry>
</row>
<row>
<entry><structfield>usename</></entry>
<entry><type>name</></entry>
<entry>Name of the user logged into this WAL sender process</entry>
</row>
<row>
<entry><structfield>application_name</></entry>
<entry><type>text</></entry>
<entry>Name of the application that is connected
to this WAL sender</entry>
</row>
<row>
<entry><structfield>client_addr</></entry>
<entry><type>inet</></entry>
<entry>IP address of the client connected to this WAL sender.
If this field is null, it indicates that the client is
connected via a Unix socket on the server machine.
</entry>
</row>
<row>
<entry><structfield>client_hostname</></entry>
<entry><type>text</></entry>
<entry>Host name of the connected client, as reported by a
reverse DNS lookup of <structfield>client_addr</>. This field will
only be non-null for IP connections, and only when <xref
linkend="guc-log-hostname"> is enabled.
</entry>
</row>
<row>
<entry><structfield>client_port</></entry>
<entry><type>integer</></entry>
<entry>TCP port number that the client is using for communication
with this WAL sender, or <literal>-1</> if a Unix socket is used
</entry>
</row>
<row>
<entry><structfield>backend_start</></entry>
<entry><type>timestamp with time zone</></entry>
<entry>Time when this process was started, i.e., when the
client connected to this WAL sender
</entry>
</row>
<row>
<entry><structfield>state</></entry>
<entry><type>text</></entry>
<entry>Current WAL sender state</entry>
</row>
<row>
<entry><structfield>sent_location</></entry>
<entry><type>text</></entry>
<entry>Last transaction log position sent on this connection</entry>
</row>
<row>
<entry><structfield>write_location</></entry>
<entry><type>text</></entry>
<entry>Last transaction log position written to disk by this standby
server</entry>
</row>
<row>
<entry><structfield>flush_location</></entry>
<entry><type>text</></entry>
<entry>Last transaction log position flushed to disk by this standby
server</entry>
</row>
<row>
<entry><structfield>replay_location</></entry>
<entry><type>text</></entry>
<entry>Last transaction log position replayed into the database on this
standby server</entry>
</row>
<row>
<entry><structfield>sync_priority</></entry>
<entry><type>integer</></entry>
<entry>Priority of this standby server for being chosen as the
synchronous standby</entry>
</row>
<row>
<entry><structfield>sync_state</></entry>
<entry><type>text</></entry>
<entry>Synchronous state of this standby server</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_replication</structname> view will contain one row
per WAL sender process, showing statistics about replication to that
sender's connected standby server. Only directly connected standbys are
listed; no information is available about downstream standby servers.
</para>
<table id="pg-stat-database-conflicts-view" xreflabel="pg_stat_database_conflicts">
<title><structname>pg_stat_database_conflicts</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>datid</></entry>
<entry><type>oid</></entry>
<entry>OID of a database</entry>
</row>
<row>
<entry><structfield>datname</></entry>
<entry><type>name</></entry>
<entry>Name of this database</entry>
</row>
<row>
<entry><structfield>confl_tablespace</></entry>
<entry><type>bigint</></entry>
<entry>Number of queries in this database that have been canceled due to
dropped tablespaces</entry>
</row>
<row>
<entry><structfield>confl_lock</></entry>
<entry><type>bigint</></entry>
<entry>Number of queries in this database that have been canceled due to
lock timeouts</entry>
</row>
<row>
<entry><structfield>confl_snapshot</></entry>
<entry><type>bigint</></entry>
<entry>Number of queries in this database that have been canceled due to
old snapshots</entry>
</row>
<row>
<entry><structfield>confl_bufferpin</></entry>
<entry><type>bigint</></entry>
<entry>Number of queries in this database that have been canceled due to
pinned buffers</entry>
</row>
<row>
<entry><structfield>confl_deadlock</></entry>
<entry><type>bigint</></entry>
<entry>Number of queries in this database that have been canceled due to
deadlocks</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_database_conflicts</structname> view will contain
one row per database, showing database-wide statistics about
query cancels occurring due to conflicts with recovery on standby servers.
This view will only contain information on standby servers, since
conflicts do not occur on master servers.
</para>
</sect2>
<sect2 id="monitoring-stats-functions">
<title>Statistics Functions</title>
<para>
Other ways of looking at the statistics can be set up by writing
queries that use the same underlying statistics access functions used by
the standard views shown above. For details such as the functions' names,
consult the definitions of the standard views. (For example, in
<application>gsql</> you could issue <literal>\d+ pg_stat_activity</>.)
The access functions for per-database statistics take a database OID as an
argument to identify which database to report on.
The per-table and per-index functions take a table or index OID.
The functions for per-function statistics take a function OID.
Note that only tables, indexes, and functions in the current database
can be seen with these functions.
</para>
<para>
Additional functions related to statistics collection are listed in <xref
linkend="monitoring-stats-funcs-table">.
</para>
<table id="monitoring-stats-funcs-table">
<title>Additional Statistics Functions</title>
<tgroup cols="3">
<thead>
<row>
<entry>Function</entry>
<entry>Return Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<!-- See also the entry for this in func.sgml -->
<entry><literal><function>pg_backend_pid()</function></literal></entry>
<entry><type>integer</type></entry>
<entry>
Process ID of the server process handling the current session
</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_activity</function>(<type>integer</type>)</literal></entry>
<entry><type>setof record</type></entry>
<entry>
Returns a record of information about the backend with the specified PID, or
one record for each active backend in the system if <symbol>NULL</symbol> is
specified. The fields returned are a subset of those in the
<structname>pg_stat_activity</structname> view.
</entry>
</row>
<row>
<entry><literal><function>pg_stat_clear_snapshot()</function></literal></entry>
<entry><type>void</type></entry>
<entry>
Discard the current statistics snapshot
</entry>
</row>
<row>
<entry><literal><function>pg_stat_reset()</function></literal></entry>
<entry><type>void</type></entry>
<entry>
Reset all statistics counters for the current database to zero
(requires superuser privileges)
</entry>
</row>
<row>
<entry><literal><function>pg_stat_reset_shared</function>(text)</literal></entry>
<entry><type>void</type></entry>
<entry>
Reset some cluster-wide statistics counters to zero, depending on the
argument (requires superuser privileges).
Calling <literal>pg_stat_reset_shared('bgwriter')</> will zero all the
counters shown in the <structname>pg_stat_bgwriter</> view.
</entry>
</row>
<row>
<entry><literal><function>pg_stat_reset_single_table_counters</function>(oid)</literal></entry>
<entry><type>void</type></entry>
<entry>
Reset statistics for a single table or index in the current database to
zero (requires superuser privileges)
</entry>
</row>
<row>
<entry><literal><function>pg_stat_reset_single_function_counters</function>(oid)</literal></entry>
<entry><type>void</type></entry>
<entry>
Reset statistics for a single function in the current database to
zero (requires superuser privileges)
</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
<function>pg_stat_get_activity</function>, the underlying function of
the <structname>pg_stat_activity</> view, returns a set of records
containing all the available information about each backend process.
Sometimes it may be more convenient to obtain just a subset of this
information. In such cases, an older set of per-backend statistics
access functions can be used; these are shown in <xref
linkend="monitoring-stats-backend-funcs-table">.
These access functions use a backend ID number, which ranges from one
to the number of currently active backends.
The function <function>pg_stat_get_backend_idset</function> provides a
convenient way to generate one row for each active backend for
invoking these functions. For example, to show the <acronym>PID</>s and
current queries of all backends:
<programlisting>
SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
pg_stat_get_backend_activity(s.backendid) AS query
FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS s;
</programlisting>
</para>
<table id="monitoring-stats-backend-funcs-table">
<title>Per-Backend Statistics 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_backend_idset()</function></literal></entry>
<entry><type>setof integer</type></entry>
<entry>Set of currently active backend ID numbers (from 1 to the
number of active backends)</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_activity(integer)</function></literal></entry>
<entry><type>text</type></entry>
<entry>Text of this backend's most recent query</>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_activity_start(integer)</function></literal></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time when the most recent query was started</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_client_addr(integer)</function></literal></entry>
<entry><type>inet</type></entry>
<entry>IP address of the client connected to this backend</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_client_port(integer)</function></literal></entry>
<entry><type>integer</type></entry>
<entry>TCP port number that the client is using for communication</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_dbid(integer)</function></literal></entry>
<entry><type>oid</type></entry>
<entry>OID of the database this backend is connected to</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_pid(integer)</function></literal></entry>
<entry><type>integer</type></entry>
<entry>Process ID of this backend</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_start(integer)</function></literal></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time when this process was started</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_userid(integer)</function></literal></entry>
<entry><type>oid</type></entry>
<entry>OID of the user logged into this backend</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_waiting(integer)</function></literal></entry>
<entry><type>boolean</type></entry>
<entry>True if this backend is currently waiting on a lock</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_xact_start(integer)</function></literal></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time when the current transaction was started</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
</sect1>
<sect1 id="monitoring-locks">
<title>Viewing Locks</title>
<indexterm zone="monitoring-locks">
<primary>lock</primary>
<secondary>monitoring</secondary>
</indexterm>
<para>
Another useful tool for monitoring database activity is the
<structname>pg_locks</structname> system table. It allows the
database administrator to view information about the outstanding
locks in the lock manager. For example, this capability can be used
to:
<itemizedlist>
<listitem>
<para>
View all the locks currently outstanding, all the locks on
relations in a particular database, all the locks on a
particular relation, or all the locks held by a particular
<productname>PostgreSQL</productname> session.
</para>
</listitem>
<listitem>
<para>
Determine the relation in the current database with the most
ungranted locks (which might be a source of contention among
database clients).
</para>
</listitem>
<listitem>
<para>
Determine the effect of lock contention on overall database
performance, as well as the extent to which contention varies
with overall database traffic.
</para>
</listitem>
</itemizedlist>
Details of the <structname>pg_locks</structname> view appear in
<xref linkend="view-pg-locks">.
For more information on locking and managing concurrency with
<productname>PostgreSQL</productname>, refer to <xref linkend="mvcc">.
</para>
</sect1>
<sect1 id="dynamic-trace">
<title>Dynamic Tracing</title>
<indexterm zone="dynamic-trace">
<primary>DTrace</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname> provides facilities to support
dynamic tracing of the database server. This allows an external
utility to be called at specific points in the code and thereby trace
execution.
</para>
<para>
A number of probes or trace points are already inserted into the source
code. These probes are intended to be used by database developers and
administrators. By default the probes are not compiled into
<productname>PostgreSQL</productname>; the user needs to explicitly tell
the configure script to make the probes available.
</para>
<para>
Currently, only the
<ulink url="http://opensolaris.org/os/community/dtrace/">DTrace</ulink>
utility is supported, which is available
on OpenSolaris, Solaris 10, and Mac OS X Leopard. It is expected that
DTrace will be available in the future on FreeBSD and possibly other
operating systems. The
<ulink url="http://sourceware.org/systemtap/">SystemTap</ulink> project
for Linux also provides a DTrace equivalent. Supporting other dynamic
tracing utilities is theoretically possible by changing the definitions for
the macros in <filename>src/include/utils/probes.h</>.
</para>
<sect2 id="compiling-for-trace">
<title>Compiling for Dynamic Tracing</title>
<para>
By default, probes are not available, so you will need to
explicitly tell the configure script to make the probes available
in <productname>PostgreSQL</productname>. To include DTrace support
specify <option>--enable-dtrace</> to configure. See <xref
linkend="install-procedure"> for further information.
</para>
</sect2>
<sect2 id="trace-points">
<title>Built-in Probes</title>
<para>
A number of standard probes are provided in the source code,
as shown in <xref linkend="dtrace-probe-point-table">;
<xref linkend="typedefs-table">
shows the types used in the probes. More probes can certainly be
added to enhance <productname>PostgreSQL</>'s observability.
</para>
<table id="dtrace-probe-point-table">
<title>Built-in DTrace Probes</title>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Parameters</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>transaction-start</entry>
<entry>(LocalTransactionId)</entry>
<entry>Probe that fires at the start of a new transaction.
arg0 is the transaction ID.</entry>
</row>
<row>
<entry>transaction-commit</entry>
<entry>(LocalTransactionId)</entry>
<entry>Probe that fires when a transaction completes successfully.
arg0 is the transaction ID.</entry>
</row>
<row>
<entry>transaction-abort</entry>
<entry>(LocalTransactionId)</entry>
<entry>Probe that fires when a transaction completes unsuccessfully.
arg0 is the transaction ID.</entry>
</row>
<row>
<entry>query-start</entry>
<entry>(const char *)</entry>
<entry>Probe that fires when the processing of a query is started.
arg0 is the query string.</entry>
</row>
<row>
<entry>query-done</entry>
<entry>(const char *)</entry>
<entry>Probe that fires when the processing of a query is complete.
arg0 is the query string.</entry>
</row>
<row>
<entry>query-parse-start</entry>
<entry>(const char *)</entry>
<entry>Probe that fires when the parsing of a query is started.
arg0 is the query string.</entry>
</row>
<row>
<entry>query-parse-done</entry>
<entry>(const char *)</entry>
<entry>Probe that fires when the parsing of a query is complete.
arg0 is the query string.</entry>
</row>
<row>
<entry>query-rewrite-start</entry>
<entry>(const char *)</entry>
<entry>Probe that fires when the rewriting of a query is started.
arg0 is the query string.</entry>
</row>
<row>
<entry>query-rewrite-done</entry>
<entry>(const char *)</entry>
<entry>Probe that fires when the rewriting of a query is complete.
arg0 is the query string.</entry>
</row>
<row>
<entry>query-plan-start</entry>
<entry>()</entry>
<entry>Probe that fires when the planning of a query is started.</entry>
</row>
<row>
<entry>query-plan-done</entry>
<entry>()</entry>
<entry>Probe that fires when the planning of a query is complete.</entry>
</row>
<row>
<entry>query-execute-start</entry>
<entry>()</entry>
<entry>Probe that fires when the execution of a query is started.</entry>
</row>
<row>
<entry>query-execute-done</entry>
<entry>()</entry>
<entry>Probe that fires when the execution of a query is complete.</entry>
</row>
<row>
<entry>statement-status</entry>
<entry>(const char *)</entry>
<entry>Probe that fires anytime the server process updates its
<structname>pg_stat_activity</>.<structfield>status</>.
arg0 is the new status string.</entry>
</row>
<row>
<entry>checkpoint-start</entry>
<entry>(int)</entry>
<entry>Probe that fires when a checkpoint is started.
arg0 holds the bitwise flags used to distinguish different checkpoint
types, such as shutdown, immediate or force.</entry>
</row>
<row>
<entry>checkpoint-done</entry>
<entry>(int, int, int, int, int)</entry>
<entry>Probe that fires when a checkpoint is complete.
(The probes listed next fire in sequence during checkpoint processing.)
arg0 is the number of buffers written. arg1 is the total number of
buffers. arg2, arg3 and arg4 contain the number of xlog file(s) added,
removed and recycled respectively.</entry>
</row>
<row>
<entry>clog-checkpoint-start</entry>
<entry>(bool)</entry>
<entry>Probe that fires when the CLOG portion of a checkpoint is started.
arg0 is true for normal checkpoint, false for shutdown
checkpoint.</entry>
</row>
<row>
<entry>clog-checkpoint-done</entry>
<entry>(bool)</entry>
<entry>Probe that fires when the CLOG portion of a checkpoint is
complete. arg0 has the same meaning as for clog-checkpoint-start.</entry>
</row>
<row>
<entry>subtrans-checkpoint-start</entry>
<entry>(bool)</entry>
<entry>Probe that fires when the SUBTRANS portion of a checkpoint is
started.
arg0 is true for normal checkpoint, false for shutdown
checkpoint.</entry>
</row>
<row>
<entry>subtrans-checkpoint-done</entry>
<entry>(bool)</entry>
<entry>Probe that fires when the SUBTRANS portion of a checkpoint is
complete. arg0 has the same meaning as for
subtrans-checkpoint-start.</entry>
</row>
<row>
<entry>multixact-checkpoint-start</entry>
<entry>(bool)</entry>
<entry>Probe that fires when the MultiXact portion of a checkpoint is
started.
arg0 is true for normal checkpoint, false for shutdown
checkpoint.</entry>
</row>
<row>
<entry>multixact-checkpoint-done</entry>
<entry>(bool)</entry>
<entry>Probe that fires when the MultiXact portion of a checkpoint is
complete. arg0 has the same meaning as for
multixact-checkpoint-start.</entry>
</row>
<row>
<entry>buffer-checkpoint-start</entry>
<entry>(int)</entry>
<entry>Probe that fires when the buffer-writing portion of a checkpoint
is started.
arg0 holds the bitwise flags used to distinguish different checkpoint
types, such as shutdown, immediate or force.</entry>
</row>
<row>
<entry>buffer-sync-start</entry>
<entry>(int, int)</entry>
<entry>Probe that fires when we begin to write dirty buffers during
checkpoint (after identifying which buffers must be written).
arg0 is the total number of buffers.
arg1 is the number that are currently dirty and need to be written.</entry>
</row>
<row>
<entry>buffer-sync-written</entry>
<entry>(int)</entry>
<entry>Probe that fires after each buffer is written during checkpoint.
arg0 is the ID number of the buffer.</entry>
</row>
<row>
<entry>buffer-sync-done</entry>
<entry>(int, int, int)</entry>
<entry>Probe that fires when all dirty buffers have been written.
arg0 is the total number of buffers.
arg1 is the number of buffers actually written by the checkpoint process.
arg2 is the number that were expected to be written (arg1 of
buffer-sync-start); any difference reflects other processes flushing
buffers during the checkpoint.</entry>
</row>
<row>
<entry>buffer-checkpoint-sync-start</entry>
<entry>()</entry>
<entry>Probe that fires after dirty buffers have been written to the
kernel, and before starting to issue fsync requests.</entry>
</row>
<row>
<entry>buffer-checkpoint-done</entry>
<entry>()</entry>
<entry>Probe that fires when syncing of buffers to disk is
complete.</entry>
</row>
<row>
<entry>twophase-checkpoint-start</entry>
<entry>()</entry>
<entry>Probe that fires when the two-phase portion of a checkpoint is
started.</entry>
</row>
<row>
<entry>twophase-checkpoint-done</entry>
<entry>()</entry>
<entry>Probe that fires when the two-phase portion of a checkpoint is
complete.</entry>
</row>
<row>
<entry>buffer-read-start</entry>
<entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool)</entry>
<entry>Probe that fires when a buffer read is started.
arg0 and arg1 contain the fork and block numbers of the page (but
arg1 will be -1 if this is a relation extension request).
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or InvalidBackendId (-1) for a shared buffer.
arg6 is true for a relation extension request, false for normal
read.</entry>
</row>
<row>
<entry>buffer-read-done</entry>
<entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool, bool)</entry>
<entry>Probe that fires when a buffer read is complete.
arg0 and arg1 contain the fork and block numbers of the page (if this
is a relation extension request, arg1 now contains the block number
of the newly added block).
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or InvalidBackendId (-1) for a shared buffer.
arg6 is true for a relation extension request, false for normal
read.
arg7 is true if the buffer was found in the pool, false if not.</entry>
</row>
<row>
<entry>buffer-flush-start</entry>
<entry>(ForkNumber, BlockNumber, Oid, Oid, Oid)</entry>
<entry>Probe that fires before issuing any write request for a shared
buffer.
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.</entry>
</row>
<row>
<entry>buffer-flush-done</entry>
<entry>(ForkNumber, BlockNumber, Oid, Oid, Oid)</entry>
<entry>Probe that fires when a write request is complete. (Note
that this just reflects the time to pass the data to the kernel;
it's typically not actually been written to disk yet.)
The arguments are the same as for buffer-flush-start.</entry>
</row>
<row>
<entry>buffer-write-dirty-start</entry>
<entry>(ForkNumber, BlockNumber, Oid, Oid, Oid)</entry>
<entry>Probe that fires when a server process begins to write a dirty
buffer. (If this happens often, it implies that
<xref linkend="guc-shared-buffers"> is too
small or the bgwriter control parameters need adjustment.)
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.</entry>
</row>
<row>
<entry>buffer-write-dirty-done</entry>
<entry>(ForkNumber, BlockNumber, Oid, Oid, Oid)</entry>
<entry>Probe that fires when a dirty-buffer write is complete.
The arguments are the same as for buffer-write-dirty-start.</entry>
</row>
<row>
<entry>wal-buffer-write-dirty-start</entry>
<entry>()</entry>
<entry>Probe that fires when a server process begins to write a
dirty WAL buffer because no more WAL buffer space is available.
(If this happens often, it implies that
<xref linkend="guc-wal-buffers"> is too small.)</entry>
</row>
<row>
<entry>wal-buffer-write-dirty-done</entry>
<entry>()</entry>
<entry>Probe that fires when a dirty WAL buffer write is complete.</entry>
</row>
<row>
<entry>xlog-insert</entry>
<entry>(unsigned char, unsigned char)</entry>
<entry>Probe that fires when a WAL record is inserted.
arg0 is the resource manager (rmid) for the record.
arg1 contains the info flags.</entry>
</row>
<row>
<entry>xlog-switch</entry>
<entry>()</entry>
<entry>Probe that fires when a WAL segment switch is requested.</entry>
</row>
<row>
<entry>smgr-md-read-start</entry>
<entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</entry>
<entry>Probe that fires when beginning to read a block from a relation.
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or InvalidBackendId (-1) for a shared buffer.</entry>
</row>
<row>
<entry>smgr-md-read-done</entry>
<entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</entry>
<entry>Probe that fires when a block read is complete.
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or InvalidBackendId (-1) for a shared buffer.
arg6 is the number of bytes actually read, while arg7 is the number
requested (if these are different it indicates trouble).</entry>
</row>
<row>
<entry>smgr-md-write-start</entry>
<entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</entry>
<entry>Probe that fires when beginning to write a block to a relation.
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or InvalidBackendId (-1) for a shared buffer.</entry>
</row>
<row>
<entry>smgr-md-write-done</entry>
<entry>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</entry>
<entry>Probe that fires when a block write is complete.
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or InvalidBackendId (-1) for a shared buffer.
arg6 is the number of bytes actually written, while arg7 is the number
requested (if these are different it indicates trouble).</entry>
</row>
<row>
<entry>sort-start</entry>
<entry>(int, bool, int, int, bool)</entry>
<entry>Probe that fires when a sort operation is started.
arg0 indicates heap, index or datum sort.
arg1 is true for unique-value enforcement.
arg2 is the number of key columns.
arg3 is the number of kilobytes of work memory allowed.
arg4 is true if random access to the sort result is required.</entry>
</row>
<row>
<entry>sort-done</entry>
<entry>(bool, long)</entry>
<entry>Probe that fires when a sort is complete.
arg0 is true for external sort, false for internal sort.
arg1 is the number of disk blocks used for an external sort,
or kilobytes of memory used for an internal sort.</entry>
</row>
<row>
<entry>lwlock-acquire</entry>
<entry>(LWLockId, LWLockMode)</entry>
<entry>Probe that fires when an LWLock has been acquired.
arg0 is the LWLock's ID.
arg1 is the requested lock mode, either exclusive or shared.</entry>
</row>
<row>
<entry>lwlock-release</entry>
<entry>(LWLockId)</entry>
<entry>Probe that fires when an LWLock has been released (but note
that any released waiters have not yet been awakened).
arg0 is the LWLock's ID.</entry>
</row>
<row>
<entry>lwlock-wait-start</entry>
<entry>(LWLockId, LWLockMode)</entry>
<entry>Probe that fires when an LWLock was not immediately available and
a server process has begun to wait for the lock to become available.
arg0 is the LWLock's ID.
arg1 is the requested lock mode, either exclusive or shared.</entry>
</row>
<row>
<entry>lwlock-wait-done</entry>
<entry>(LWLockId, LWLockMode)</entry>
<entry>Probe that fires when a server process has been released from its
wait for an LWLock (it does not actually have the lock yet).
arg0 is the LWLock's ID.
arg1 is the requested lock mode, either exclusive or shared.</entry>
</row>
<row>
<entry>lwlock-condacquire</entry>
<entry>(LWLockId, LWLockMode)</entry>
<entry>Probe that fires when an LWLock was successfully acquired when the
caller specified no waiting.
arg0 is the LWLock's ID.
arg1 is the requested lock mode, either exclusive or shared.</entry>
</row>
<row>
<entry>lwlock-condacquire-fail</entry>
<entry>(LWLockId, LWLockMode)</entry>
<entry>Probe that fires when an LWLock was not successfully acquired when
the caller specified no waiting.
arg0 is the LWLock's ID.
arg1 is the requested lock mode, either exclusive or shared.</entry>
</row>
<row>
<entry>lock-wait-start</entry>
<entry>(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</entry>
<entry>Probe that fires when a request for a heavyweight lock (lmgr lock)
has begun to wait because the lock is not available.
arg0 through arg3 are the tag fields identifying the object being
locked. arg4 indicates the type of object being locked.
arg5 indicates the lock type being requested.</entry>
</row>
<row>
<entry>lock-wait-done</entry>
<entry>(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</entry>
<entry>Probe that fires when a request for a heavyweight lock (lmgr lock)
has finished waiting (i.e., has acquired the lock).
The arguments are the same as for lock-wait-start.</entry>
</row>
<row>
<entry>deadlock-found</entry>
<entry>()</entry>
<entry>Probe that fires when a deadlock is found by the deadlock
detector.</entry>
</row>
</tbody>
</tgroup>
</table>
<table id="typedefs-table">
<title>Defined Types Used in Probe Parameters</title>
<tgroup cols="2">
<thead>
<row>
<entry>Type</entry>
<entry>Definition</entry>
</row>
</thead>
<tbody>
<row>
<entry>LocalTransactionId</entry>
<entry>unsigned int</entry>
</row>
<row>
<entry>LWLockId</entry>
<entry>int</entry>
</row>
<row>
<entry>LWLockMode</entry>
<entry>int</entry>
</row>
<row>
<entry>LOCKMODE</entry>
<entry>int</entry>
</row>
<row>
<entry>BlockNumber</entry>
<entry>unsigned int</entry>
</row>
<row>
<entry>Oid</entry>
<entry>unsigned int</entry>
</row>
<row>
<entry>ForkNumber</entry>
<entry>int</entry>
</row>
<row>
<entry>bool</entry>
<entry>char</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2 id="using-trace-points">
<title>Using Probes</title>
<para>
The example below shows a DTrace script for analyzing transaction
counts in the system, as an alternative to snapshotting
<structname>pg_stat_database</> before and after a performance test:
<programlisting>
#!/usr/sbin/dtrace -qs
postgresql$1:::transaction-start
{
@start["Start"] = count();
self->ts = timestamp;
}
postgresql$1:::transaction-abort
{
@abort["Abort"] = count();
}
postgresql$1:::transaction-commit
/self->ts/
{
@commit["Commit"] = count();
@time["Total time (ns)"] = sum(timestamp - self->ts);
self->ts=0;
}
</programlisting>
When executed, the example D script gives output such as:
<screen>
# ./txn_count.d `pgrep -n gaussdb` or ./txn_count.d <PID>
^C
Start 71
Commit 70
Total time (ns) 2312105013
</screen>
</para>
<note>
<para>
SystemTap uses a different notation for trace scripts than DTrace does,
even though the underlying trace points are compatible. One point worth
noting is that at this writing, SystemTap scripts must reference probe
names using double underscores in place of hyphens. This is expected to
be fixed in future SystemTap releases.
</para>
</note>
<para>
You should remember that DTrace scripts need to be carefully written and
debugged, otherwise the trace information collected might
be meaningless. In most cases where problems are found it is the
instrumentation that is at fault, not the underlying system. When
discussing information found using dynamic tracing, be sure to enclose
the script used to allow that too to be checked and discussed.
</para>
<para>
More example scripts can be found in the PgFoundry
<ulink url="http://pgfoundry.org/projects/dtrace/">dtrace project</ulink>.
</para>
</sect2>
<sect2 id="defining-trace-points">
<title>Defining New Probes</title>
<para>
New probes can be defined within the code wherever the developer
desires, though this will require a recompilation. Below are the steps
for inserting new probes:
</para>
<procedure>
<step>
<para>
Decide on probe names and data to be made available through the probes
</para>
</step>
<step>
<para>
Add the probe definitions to <filename>src/backend/utils/probes.d</>
</para>
</step>
<step>
<para>
Include <filename>pg_trace.h</> if it is not already present in the
module(s) containing the probe points, and insert
<literal>TRACE_POSTGRESQL</> probe macros at the desired locations
in the source code
</para>
</step>
<step>
<para>
Recompile and verify that the new probes are available
</para>
</step>
</procedure>
<formalpara>
<title>Example:</title>
<para>
Here is an example of how you would add a probe to trace all new
transactions by transaction ID.
</para>
</formalpara>
<procedure>
<step>
<para>
Decide that the probe will be named <literal>transaction-start</> and
requires a parameter of type LocalTransactionId
</para>
</step>
<step>
<para>
Add the probe definition to <filename>src/backend/utils/probes.d</>:
<programlisting>
probe transaction__start(LocalTransactionId);
</programlisting>
Note the use of the double underline in the probe name. In a DTrace
script using the probe, the double underline needs to be replaced with a
hyphen, so <literal>transaction-start</> is the name to document for
users.
</para>
</step>
<step>
<para>
At compile time, <literal>transaction__start</> is converted to a macro
called <literal>TRACE_POSTGRESQL_TRANSACTION_START</> (notice the
underscores are single here), which is available by including
<filename>pg_trace.h</>. Add the macro call to the appropriate location
in the source code. In this case, it looks like the following:
<programlisting>
TRACE_POSTGRESQL_TRANSACTION_START(vxid.localTransactionId);
</programlisting>
</para>
</step>
<step>
<para>
After recompiling and running the new binary, check that your newly added
probe is available by executing the following DTrace command. You
should see similar output:
<screen>
# dtrace -ln transaction-start
ID PROVIDER MODULE FUNCTION NAME
18705 postgresql49878 gaussdb StartTransactionCommand transaction-start
18755 postgresql49877 gaussdb StartTransactionCommand transaction-start
18805 postgresql49876 gaussdb StartTransactionCommand transaction-start
18855 postgresql49875 gaussdb StartTransactionCommand transaction-start
18986 postgresql49873 gaussdb StartTransactionCommand transaction-start
</screen>
</para>
</step>
</procedure>
<para>
There are a few things to be careful about when adding trace macros
to the C code:
<itemizedlist>
<listitem>
<para>
You should take care that the data types specified for a probe's
parameters match the data types of the variables used in the macro.
Otherwise, you will get compilation errors.
</para>
</listitem>
<listitem>
<para>
On most platforms, if <productname>PostgreSQL</productname> is
built with <option>--enable-dtrace</>, the arguments to a trace
macro will be evaluated whenever control passes through the
macro, <emphasis>even if no tracing is being done</>. This is
usually not worth worrying about if you are just reporting the
values of a few local variables. But beware of putting expensive
function calls into the arguments. If you need to do that,
consider protecting the macro with a check to see if the trace
is actually enabled:
<programlisting>
if (TRACE_POSTGRESQL_TRANSACTION_START_ENABLED())
TRACE_POSTGRESQL_TRANSACTION_START(some_function(...));
</programlisting>
Each trace macro has a corresponding <literal>ENABLED</> macro.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
</sect1>
</chapter>