Re: merge semi join cost calculation error

po 8. 10. 2018 v 17:00 odesílatel Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> napsal:

> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> > The user sent a plan:
>
> > QUERY PLAN
> > Merge Semi Join (cost=82.97..580.24 rows=580 width=8) (actual
> > time=0.503..9557.396 rows=721 loops=1)
> > Merge Cond: (tips.users_id = follows.users_id_to)
> > -> Index Scan using tips_idx_users_id01 on tips
> (cost=0.43..8378397.19
> > rows=2491358 width=16) (actual time=0.009..9231.585 rows=2353914 loops=1)
> > -> Sort (cost=1.77..1.82 rows=22 width=8) (actual time=0.052..0.089
> > rows=28 loops=1)
> > Sort Key: follows.users_id_to
> > Sort Method: quicksort Memory: 26kB
> > -> Seq Scan on follows (cost=0.00..1.27 rows=22 width=8)
> (actual
> > time=0.013..0.020 rows=28 loops=1)
> > Filter: (users_id_from = 1)
>
> > He has PostgreSQL 10.5. I cannot to understand to too low total cost of
> Merge
> > Semi Join because subnode has very high cost 8378397.
>
> The planner seems to be supposing that the merge will stop far short of
> scanning the entire LHS table, presumably as a result of thinking that
> the maximum value of follows.users_id_to is much less than the maximum
> value of tips.users_id. Given the actual rowcounts, that's seemingly
> not true, which suggests out-of-date stats for one table or the other.
>

good tip - the table follows was too small for autovacuum, and it was
terrible effect. I didn't know about this optimization.

Thank you

Pavel

>
> regards, tom lane
>