PostgreSQL'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 query currently being executed by other server
processes.
Since collection of statistics adds some overhead to query execution,
the system can be configured to collect or not collect information.
This is controlled by configuration variables that are normally set in
postgresql.conf (see Section 3.4 for
details about setting configuration variables).
The variable STATS_START_COLLECTOR 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 variables STATS_COMMAND_STRING,
STATS_BLOCK_LEVEL,
and STATS_ROW_LEVEL control how much information is
actually sent to the collector, and thus determine how much run-time
overhead occurs. These respectively determine whether a server process
sends its current command string, disk-block-level access statistics, and
row-level access statistics to the collector. Normally these variables 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 server
processes using the SET command. (To prevent ordinary users
from hiding their activity from the administrator, only superusers are
allowed to change these variables with SET.)
Important: Since the variables STATS_COMMAND_STRING,
STATS_BLOCK_LEVEL,
and STATS_ROW_LEVEL
default to false, no statistics are actually collected
in the default configuration. You must turn one or more of them on
before you will get useful results from the statistical display
functions.
Several predefined views are available to show the results of
statistics collection, listed in Table 10-1. 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. It then continues to use this snapshot for all
statistical views and functions until the end of its current transaction.
So the statistics will appear not to change as long as you continue the
current transaction.
This is a feature, not a bug, because it allows you to perform several
queries on the statistics and correlate the results without worrying that
the numbers are changing underneath you. But if you want to see new
results with each query, be sure to do the queries outside any transaction
block.
Table 10-1. Standard Statistics Views
View Name
Description
pg_stat_activity
One row per server process, showing process ID, database,
user, and current query. The current query column is only available
to superusers; for others it reads as null. (Note that because of
the collector's reporting delay, current query will only be up-to-date
for long-running queries.)
pg_stat_database
One row per database, showing number of active backends,
total transactions committed and total rolled back in that database,
total disk blocks read, and total number of buffer hits (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 tuples returned by
each type of scan, and totals of tuple insertions, updates,
and deletes.
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 tuples
read, and the number of successfully fetched heap tuples. (This may
be less when there are index entries pointing to expired heap tuples.)
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.
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 Table 10-2. The per-database access
functions accept a database OID to identify which database to
report on. The per-table and per-index functions accept a table or
index OID (note that only tables and indexes in the current
database can be seen with these functions). The per-backend access
functions accept a backend ID number, which ranges from one to the
number of currently active backends.
Table 10-2. Statistics Access Functions
Function
Return Type
Description
pg_stat_get_db_numbackends(oid)
integer
Number of active backends in 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 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 tuples read by sequential scans when argument is a table,
or number of index tuples read when argument is an index
pg_stat_get_tuples_fetched(oid)
bigint
Number of valid (unexpired) table tuples fetched by sequential scans
when argument is a table, or fetched by index scans using this index
when argument is an index
pg_stat_get_tuples_inserted(oid)
bigint
Number of tuples inserted into table
pg_stat_get_tuples_updated(oid)
bigint
Number of tuples updated in table
pg_stat_get_tuples_deleted(oid)
bigint
Number of tuples 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 IDs (from 1 to N where N is the
number of active backends). See usage example below.
pg_backend_pid()
integer
Process ID of the attached backend
pg_stat_get_backend_pid(integer)
integer
Process ID of all backend processes
pg_stat_get_backend_dbid(integer)
oid
Database ID of backend process
pg_stat_get_backend_userid(integer)
oid
User ID of backend process
pg_stat_get_backend_activity(integer)
text
Current query of backend process (NULL if caller is not superuser)
pg_stat_reset()
boolean
Reset all currently collected statistics.
Note: Blocks_fetched minus blocks_hit gives the number of kernel
read() calls issued for the table, index, or
database; but the actual number of physical reads is usually
lower due to kernel-level buffering.
The function pg_stat_get_backend_idset provides
a convenient way to generate one row for each active backend. For
example, to show the PIDs and current queries of all backends:
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;
