Tomas Vondra wrote:
Stats about access to the data (index/seq scans, cache hit ratio etc.) are stored in pg_stat_* and pg_statio_* catalogs, and are updated after running each query. AFAIK it's not a synchronous process, but when a backend finishes a query, it sends the stats to the postmaster (and postmaster updates the catalogs).
Description in the docs goes over this in a little more detail http://www.postgresql.org/docs/current/static/monitoring-stats.html :
"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 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 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."
It's not synchronous at all. The clients create a temporary file for the statistics collector and move on. The actual statistics don't get updated until the statistics collector decides enough time has passed to bother, which defaults to at most every 500ms.
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