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 runtime 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. 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 (500 milliseconds 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 PID, database, user, and current query. The current query column is only available to superusers; for others it reads as NULL. (Note that because of the collector's reporting delay, current query will only be up-to-date for long-running queries.) |
| 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 (ie, 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. 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_stat_get_backend_pid(integer) | integer | PID of backend process |
| 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) |
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;