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23.2. The Statistics Collector

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 command currently being executed by other server processes.

23.2.1. 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 Section 16.4 for details about setting configuration parameters.)

The parameter 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 parameters 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 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 SET command. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.)

Note: 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.

23.2.2. Viewing Collected Statistics

Several predefined views, listed in Table 23-1, 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 block and row access 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 pgstat_stat_interval milliseconds (500 by default). So the displayed information lags behind actual activity. Current-query information is reported to the collector immediately, but is still subject to the pgstat_stat_interval delay before it becomes visible.

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 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 23-1. Standard Statistics Views

View Name Description
pg_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 Table 23-2. 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.

Table 23-2. Statistics Access Functions

Function Return Type Description
pg_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

Note: 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 PIDs 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;
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