The Statistics Collector

PostgreSQL 7.4.18 Documentation
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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 are available to show the results of statistics collection, listed in Table 23-1. 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 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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