Monitoring Database Activitymonitoringdatabase activitydatabase activitymonitoring
A database administrator frequently wonders, What is the system
doing right now?
This chapter discusses how to find that out.
Several tools are available for monitoring database activity and
analyzing performance. Most of this chapter is devoted to describing
PostgreSQL's statistics collector,
but one should not neglect regular Unix monitoring programs such as
ps> and top>. Also, once one has identified a
poorly-performing query, further investigation may be needed using
PostgreSQL's command.
discusses EXPLAIN>
and other methods for understanding the behavior of an individual
query.
Standard Unix Toolspsto monitor activity
On most platforms, PostgreSQL modifies its
command title as reported by ps>, so that individual server
processes can readily be identified. A sample display is
$ 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
(The appropriate invocation of 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
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
postgres: user> database> host> activity>
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 idle> (i.e., waiting for a client command),
idle in transaction> (waiting for client inside a BEGIN> block),
or a command type name such as SELECT>. Also,
waiting> is attached if the server process is presently waiting
on a lock held by another server process. In the above example we can infer
that process 1003 is waiting for process 1016 to complete its transaction and
thereby release some lock or other.
Solaris requires special handling. You must
use /usr/ucb/ps, rather than
/bin/ps. You also must use two
flags, not just one. In addition, your original invocation of the
postmaster command must have a shorter
ps status display than that provided by each
server process. If you fail to do all three things, the ps>
output for each server process will be the original postmaster>
command line.
The Statistics CollectorstatisticsPostgreSQL's statistics collector>
is a subsystem that supports collection and reporting of information about
server activity. Presently, the collector can count accesses to tables
and indexes in both disk-block and individual-row terms. It also supports
determining the exact command currently being executed by other server
processes.
Statistics Collection Configuration
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
postgresql.conf>. (See for
details about setting configuration parameters.)
The parameter must be
set to 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.
The parameters ,
, and control how much information is
actually sent to the collector and thus determine how much run-time
overhead occurs. These respectively determine whether a server
process sends its current command string, disk-block-level access
statistics, and row-level access statistics to the collector.
Normally these parameters are set in 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 command. (To prevent
ordinary users from hiding their activity from the administrator,
only superusers are allowed to change these parameters with
SET>.)
Since the parameters stats_command_string,
stats_block_level, and
stats_row_level default to false>,
very few statistics are collected in the default
configuration. Enabling one or more of these configuration
variables will significantly enhance the amount of useful data
produced by the statistics collector, at the expense of
additional run-time overhead.
Viewing Collected Statistics
Several predefined views are available to show the results of
statistics collection, listed in . Alternatively, one can
build custom views using the underlying statistics functions.
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 pgstat_stat_interval milliseconds
(500 by default). So the displayed totals lag behind actual activity.
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 and then continues to use this snapshot for all
statistical views and functions until the end of its current transaction.
So the statistics will appear not to change as long as you continue the
current transaction.
This is a feature, not a bug, because it allows you to perform several
queries on the statistics and correlate the results without worrying that
the numbers are changing underneath you. But if you want to see new
results with each query, be sure to do the queries outside any transaction
block.
Standard Statistics ViewsView NameDescriptionpg_stat_activity>One row per server process, showing process
ID>, database, user, current query, and the time at
which the current query began execution. The columns that report
data on the current query are only available if the parameter
stats_command_string has been turned on.
Furthermore, these columns read as null unless the user examining
the view is a superuser or the same as the user owning the process
being reported on. (Note that because of the
collector's reporting delay, current query will only be up-to-date for
long-running queries.)pg_stat_database>One row per database, showing the number of active backend server processes,
total transactions committed and total rolled back in that database,
total disk blocks read, and total number of buffer hits (i.e., block
read requests avoided by finding the block already in buffer cache).
pg_stat_all_tables>For each table in the current database, total numbers of
sequential and index scans, total numbers of rows returned by
each type of scan, and totals of row insertions, updates,
and deletions.pg_stat_sys_tables>Same as pg_stat_all_tables>, except that only system tables
are shown.pg_stat_user_tables>Same as pg_stat_all_tables>, except that only user tables
are shown.pg_stat_all_indexes>For each index in the current database, the total number
of index scans that have used that index, the number of index rows
read, and the number of successfully fetched heap rows. (This may
be less when there are index entries pointing to expired heap rows.)
pg_stat_sys_indexes>Same as pg_stat_all_indexes>, except that only indexes on
system tables are shown.pg_stat_user_indexes>Same as pg_stat_all_indexes>, except that only indexes on
user tables are shown.pg_statio_all_tables>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.
pg_statio_sys_tables>Same as pg_statio_all_tables>, except that only system tables
are shown.pg_statio_user_tables>Same as pg_statio_all_tables>, except that only user tables
are shown.pg_statio_all_indexes>For each index in the current database, the numbers of
disk blocks read and buffer hits in that index.
pg_statio_sys_indexes>Same as pg_statio_all_indexes>, except that only indexes on
system tables are shown.pg_statio_user_indexes>Same as pg_statio_all_indexes>, except that only indexes on
user tables are shown.pg_statio_all_sequences>For each sequence object in the current database, the numbers
of disk blocks read and buffer hits in that sequence.
pg_statio_sys_sequences>Same as pg_statio_all_sequences>, except that only system
sequences are shown. (Presently, no system sequences are defined,
so this view is always empty.)pg_statio_user_sequences>Same as pg_statio_all_sequences>, except that only user
sequences are shown.
The per-index statistics are particularly useful to determine which
indexes are being used and how effective they are.
The 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 PostgreSQL>
handles disk I/O, data that is not in the
PostgreSQL> buffer cache may still reside in the
kernel's I/O cache, and may therefore still be fetched without
requiring a physical read. Users interested in obtaining more
detailed information on PostgreSQL> I/O behavior are
advised to use the PostgreSQL> statistics collector
in combination with operating system utilities that allow insight
into the kernel's handling of I/O.
Other ways of looking at the statistics can be set up by writing
queries that use the same underlying statistics access functions as
these standard views do. These functions are listed in . The per-database access
functions take a database OID as argument to identify which
database to report on. The per-table and per-index functions take
a table or index OID. (Note that only tables and indexes in the
current database can be seen with these functions.) The
per-backend process access functions take a backend process ID
number, which ranges from one to the number of currently active
backend processes.
Statistics Access FunctionsFunctionReturn TypeDescriptionpg_stat_get_db_numbackends(oid)integer
Number of active backend processes for database
pg_stat_get_db_xact_commit(oid)bigint
Transactions committed in database
pg_stat_get_db_xact_rollback(oid)bigint
Transactions rolled back in database
pg_stat_get_db_blocks_fetched(oid)bigint
Number of disk block fetch requests for database
pg_stat_get_db_blocks_hit(oid)bigint
Number of disk block fetch requests found in cache for database
pg_stat_get_numscans(oid)bigint
Number of sequential scans done when argument is a table,
or number of index scans done when argument is an index
pg_stat_get_tuples_returned(oid)bigint
Number of rows read by sequential scans when argument is a table,
or number of index rows read when argument is an index
pg_stat_get_tuples_fetched(oid)bigint
Number of valid (unexpired) table rows fetched by sequential scans
when argument is a table, or fetched by index scans using this index
when argument is an index
pg_stat_get_tuples_inserted(oid)bigint
Number of rows inserted into table
pg_stat_get_tuples_updated(oid)bigint
Number of rows updated in table
pg_stat_get_tuples_deleted(oid)bigint
Number of rows deleted from table
pg_stat_get_blocks_fetched(oid)bigint
Number of disk block fetch requests for table or index
pg_stat_get_blocks_hit(oid)bigint
Number of disk block requests found in cache for table or index
pg_stat_get_backend_idset()set of integer
Set of currently active backend process IDs (from 1 to the
number of active backend processes). See usage example in the text.
pg_backend_pid()integer
Process ID of the backend process attached to the current session
pg_stat_get_backend_pid(integer)integer
Process ID of the given backend process
pg_stat_get_backend_dbid(integer)oid
Database ID of the given backend process
pg_stat_get_backend_userid(integer)oid
User ID of the given backend process
pg_stat_get_backend_activity(integer)text
Active command of the given backend process (null if the
current user is not a superuser nor the same user as that of
the session being queried, or
stats_command_string is not on)
pg_stat_get_backend_activity_start(integer)timestamp with time zone
The time at which the given backend process' currently
executing query was started (null if the
current user is not a superuser nor the same user as that of
the session being queried, or
stats_command_string is not on)
pg_stat_reset()boolean
Reset all currently collected statistics
pg_stat_get_db_blocks_fetched minus
pg_stat_get_db_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.
The function pg_stat_get_backend_idset provides
a convenient way to generate one row for each active backend process. For
example, to show the PID>s and current queries of all backend processes:
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;
Viewing Lockslockmonitoring
Another useful tool for monitoring database activity is the
pg_locks 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:
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
PostgreSQL session.
Determine the relation in the current database with the most
ungranted locks (which might be a source of contention among
database clients).
Determine the effect of lock contention on overall database
performance, as well as the extent to which contention varies
with overall database traffic.
Details of the pg_locks view appear in
.
For more information on locking and managing concurrency with
PostgreSQL, refer to .