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13.2.1 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 4 Server Configuration for details about setting configuration parameters.)

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_activities enables monitoring of the current command being executed by any server process.

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 communicates with the backends needing information (including autovacuum) through temporary files. These files are stored in the ‘pg_stat_tmp’ subdirectory. When the postmaster shuts down, a permanent copy of the statistics data is stored in the ‘global’ subdirectory. For increased performance, the parameter stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements.