27.2.1. 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 Chapter 18 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 transmits the collected information to backends (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.