On 3/28/24 16:00, Alexander Lakhin wrote:
> ...
>
> Using the trick Thomas proposed in [1] (see my modification attached), I
> could reproduce the failure easily on my workstation with no specific
> conditions:
> 2024-03-28 14:05:13.792 UTC client backend[2358012]
> pg_regress/test_setup LOG: !!!ConditionalLockBufferForCleanup()
> returning false
> 2024-03-28 14:05:13.792 UTC client backend[2358012]
> pg_regress/test_setup CONTEXT: while scanning block 29 of relation
> "public.tenk2"
> 2024-03-28 14:05:13.792 UTC client backend[2358012]
> pg_regress/test_setup STATEMENT: VACUUM ANALYZE tenk2;
> ...
> relname | relpages | reltuples | autovacuum_count | autoanalyze_count
> ---------+----------+-----------+------------------+-------------------
> - tenk2 | 345 | 10000 | 0 | 0
> + tenk2 | 345 | 9996 | 0 | 0
> (1 row)
>
> So it looks to me like a possible cause of the failure, and I wonder
> whether checks for query plans should be immune to such changes or results
> of VACUUM ANALYZE should be 100% stable?
>
Yeah. I think it's good to design the data/queries in such a way that
the behavior does not flip due to minor noise like in this case.
But I'm a bit confused - how come the estimates do change at all? The
analyze simply fetches 30k rows, and tenk only has 10k of them. So we
should have *exact* numbers, and it should be exactly the same for all
the analyze runs. So how come it changes like this?
regards
--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
