Re: problem concerning vacuum/statistics & query performance

"Esger Abbink" <pgadmin(at)bumblebeast(dot)com> writes:
> if i manually VACUUM ANALYZE the db when it is pretty loaded (~300k
> rows) the planner starts using a different plan which runs acceptable
> performance wise. however after the daily cleanup it reverts back to
> the old (bad) plan.

> so it seems the nightly VACUUM does some analyzing as well?

Plain VACUUM (no ANALYZE) won't touch pg_statistic, but it does update
the pages and tuples estimates in pg_class.

> in which case the planner is using totally off-base statistics for
> most of the day, which in itself isnt bad but i'd rather have it use
> large table stats on small tables than vice versa.

You could do this in your nightly script:

-- delete lotsa stuff from mytable;
vacuum mytable;
update pg_class set reltuples = M, relpages = N where relname = 'mytable';

where M and N correspond to the peak values instead of the minima.
Kinda grotty but it'll get the job done.

regards, tom lane