On Sat, Apr 17, 2021 at 04:36:52PM +0200, Pavel Stehule wrote: > today I worked on postgres's server used for critical service. Because the > application is very specific, we had to do final tuning on production > server. I fix lot of queries, but I am not able to detect fast queries that > does full scan of middle size tables - to 1M rows. Surely I wouldn't log > all queries. Now, there are these queries with freq 10 per sec. > > Can be nice to have a possibility to set a log of queries that do full > scan and read more tuples than is specified limit or that does full scan of > specified tables. > > What do you think about the proposed feature?
Are you able to use auto_explain with auto_explain.log_min_duration ?
Unfortunately, I cannot use it. This server executes 5K queries per seconds, and I am afraid to decrease log_min_duration.
The logs are forwarded to the network and last time, when users played with it, then they had problems with the network.
I am in a situation where I know there are queries faster than 100ms, I see so there should be fullscans from pg_stat_user_tables, but I don't see the queries.
The fullscan of this table needs about 30ms and has 200K rows. So decreasing log_min_duration to this value is very risky.
Then you can search for query logs with message ~ 'Seq Scan .* \(actual time=[.0-9]* rows=[0-9]{6,} loops=[0-9]*)'
Or can you use pg_stat_all_tables.seq_scan ?
I use pg_stat_all_tables.seq_scan and I see seq scans there. But I need to know the related queries.
But it seems to me that filtering on the duration would be both a more important criteria and a more general one, than "seq scan with number of rows".
| (split_part(message, ' ', 2)::float/1000 AS duration ..) WHERE duration>2222;
