At Mon, 3 Jul 2023 15:45:52 +0200, Tomas Vondra <tomas.von...@enterprisedb.com> wrote in > So I'm wondering if pg_stat_force_next_flush() is enough - AFAICS this > only sets some flag for the *next* pgstat_report_stat() call, but how do > we know that happens before the query execution? > > Shouldn't there be something like pg_stat_flush() that actually does the > flushing, instead of just setting the flag?
The reason for the function is that pg_stat_flush() is supposed not to be called within a transaction. AFAICS pg_stat_force_next_flush() takes effect after a successfull transaction end and before the next command execution. To verify this, I put in an assertion to check that the flag gets consumed before reading of pg_stat_io (a.diff), then ran pgbench with the attached custom script. As expected, it didn't fire at all during several trials. When I wrapped all lines in t.sql within a begin-commit block, the assertion fired off immediately as a matter of course. Is there any chance concurrent backends or some other things can actually hinder the backend from reusing buffers? regards. -- Kyotaro Horiguchi NTT Open Source Software Center
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c index d743fc0b28..6529e737ea 100644 --- a/src/backend/utils/activity/pgstat.c +++ b/src/backend/utils/activity/pgstat.c @@ -225,7 +225,7 @@ static dlist_head pgStatPending = DLIST_STATIC_INIT(pgStatPending); * Force the next stats flush to happen regardless of * PGSTAT_MIN_INTERVAL. Useful in test scripts. */ -static bool pgStatForceNextFlush = false; +bool pgStatForceNextFlush = false; /* * Force-clear existing snapshot before next use when stats_fetch_consistency diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c index 2a4c8ef87f..7f60b2834a 100644 --- a/src/backend/utils/adt/pgstatfuncs.c +++ b/src/backend/utils/adt/pgstatfuncs.c @@ -1352,7 +1352,9 @@ pg_stat_get_io(PG_FUNCTION_ARGS) ReturnSetInfo *rsinfo; PgStat_IO *backends_io_stats; Datum reset_time; + extern bool pgStatForceNextFlush; + Assert(!pgStatForceNextFlush); InitMaterializedSRF(fcinfo, 0); rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
--begin; select pg_stat_force_next_flush(); select sum(reuses) from pg_stat_io; --commit;
