| PostgreSQL 9.4.26 Documentation | |||
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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 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.
PostgreSQL also supports reporting of the exact command currently being executed by other server processes. This facility is independent of the collector process.
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 Chapter 18 for details about setting configuration parameters.)
The parameter track_activities enables monitoring of the current command being executed by any server process.
The parameter track_counts controls whether statistics are collected about table and index accesses.
The parameter track_functions enables tracking of usage of user-defined functions.
The parameter track_io_timing enables monitoring of block read and write times.
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.)
The statistics collector transmits the collected information to other PostgreSQL processes through temporary files. These files are stored in the directory named by the stats_temp_directory parameter, pg_stat_tmp by default. For better performance, stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements. When the server shuts down cleanly, a permanent copy of the statistics data is stored in the pg_stat subdirectory, so that statistics can be retained across server restarts. When recovery is performed at server start (e.g. after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset.
Several predefined views, listed in Table 27-1, are available to show the results of statistics collection. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section 27.2.3.
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 PGSTAT_STAT_INTERVAL milliseconds (500 ms unless altered while building the server). So the displayed information lags behind actual activity. However, current-query information collected by track_activities is always up-to-date.
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 pg_stat_clear_snapshot(), 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.
A transaction can also see its own statistics (as yet untransmitted to the collector) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. These numbers do not act as stated above; instead they update continuously throughout the transaction.
Table 27-1. Standard Statistics Views
| View Name | Description |
|---|---|
| pg_stat_activity | One row per server process, showing information related to the current activity of that process, such as state and current query. See pg_stat_activity for details. |
| pg_stat_archiver | One row only, showing statistics about the WAL archiver process's activity. See pg_stat_archiver for details. |
| pg_stat_bgwriter | One row only, showing statistics about the background writer process's activity. See pg_stat_bgwriter for details. |
| pg_stat_database | One row per database, showing database-wide statistics. See pg_stat_database for details. |
| pg_stat_all_tables | One row for each table in the current database, showing statistics about accesses to that specific table. See pg_stat_all_tables for details. |
| 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_xact_all_tables | Similar to pg_stat_all_tables, but counts actions taken so far within the current transaction (which are not yet included in 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. |
| pg_stat_xact_sys_tables | Same as pg_stat_xact_all_tables, except that only system tables are shown. |
| pg_stat_xact_user_tables | Same as pg_stat_xact_all_tables, except that only user tables are shown. |
| pg_stat_all_indexes | One row for each index in the current database, showing statistics about accesses to that specific index. See pg_stat_all_indexes for details. |
| 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 | One row for each table in the current database, showing statistics about I/O on that specific table. See pg_statio_all_tables for details. |
| 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 | One row for each index in the current database, showing statistics about I/O on that specific index. See pg_statio_all_indexes for details. |
| 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 | One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See pg_statio_all_sequences for details. |
| 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. |
| pg_stat_user_functions | One row for each tracked function, showing statistics about executions of that function. See pg_stat_user_functions for details. |
| pg_stat_xact_user_functions | Similar to pg_stat_user_functions, but counts only calls during the current transaction (which are not yet included in pg_stat_user_functions). |
| pg_stat_replication | One row per WAL sender process, showing statistics about replication to that sender's connected standby server. See pg_stat_replication for details. |
| pg_stat_database_conflicts | One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See pg_stat_database_conflicts for details. |
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 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 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.
Table 27-2. pg_stat_activity View
| Column | Type | Description |
|---|---|---|
| datid | oid | OID of the database this backend is connected to |
| datname | name | Name of the database this backend is connected to |
| pid | integer | Process ID of this backend |
| usesysid | oid | OID of the user logged into this backend |
| usename | name | Name of the user logged into this backend |
| application_name | text | Name of the application that is connected to this backend |
| client_addr | inet | 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. |
| client_hostname | text | Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when log_hostname is enabled. |
| client_port | integer | TCP port number that the client is using for communication with this backend, or -1 if a Unix socket is used |
| backend_start | timestamp with time zone | Time when this process was started, i.e., when the client connected to the server |
| xact_start | timestamp with time zone | 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 query_start column. |
| query_start | timestamp with time zone | Time when the currently active query was started, or if state is not active, when the last query was started |
| state_change | timestamp with time zone | Time when the state was last changed |
| waiting | boolean | True if this backend is currently waiting on a lock |
| state | text | Current overall state of this backend. Possible values are:
|