Re: why query plan for the inner SELECT of WHERE x IN is wrong, but when run the inner query alone is OK?

It is hardware dependent. The estimates are not time estimates, but on an
arbitrary scale.

On the server I work with, the estimates are almost always 10x larger than
the run times, and sometimes more than 50x.

(many GBs RAM, 8 CPU cores, more than 10 disks, standard optimizer settings
other than statistics sample sizes and increased common values for columns).

-Scott

On Sat, Aug 9, 2008 at 7:20 PM, Miernik <public(at)public(dot)miernik(dot)name> wrote:

> Miernik <public(at)public(dot)miernik(dot)name> wrote:
> > I present a SELECT uid plan with the 1000 table also below, just to be
> > sure, this is the "bad" plan, that takes forever:
> >
> > miernik=> EXPLAIN SELECT uid FROM cnts WHERE uid IN (SELECT uid FROM alog
> WHERE pid = 3452654 AND o = 1);
> > QUERY PLAN
> >
> -----------------------------------------------------------------------------------------------
> > Nested Loop IN Join (cost=0.00..3532.70 rows=1 width=4)
> > -> Seq Scan on cnts (cost=0.00..26.26 rows=1026 width=4)
> > -> Index Scan using alog_uid_idx on alog (cost=0.00..297.32 rows=1
> width=4)
> > Index Cond: ((alog.uid)::integer = (cnts.uid)::integer)
> > Filter: ((alog.pid = 3452654::numeric) AND (alog.o = 1::numeric))
> > (5 rows)
>
> If I reduce the number of rows in cnts to 100, I can actually make an
> EXPLAIN ANALYZE with this query plan finish in reasonable time:
>
> miernik=> EXPLAIN ANALYZE SELECT uid FROM cnts WHERE uid IN (SELECT uid
> FROM alog WHERE pid = 555949 AND odp = 1);
> QUERY
> PLAN
>
> -------------------------------------------------------------------------------------------------------------------------------------------------
> Nested Loop IN Join (cost=0.00..3585.54 rows=1 width=4) (actual
> time=51831.430..267844.815 rows=7 loops=1)
> -> Seq Scan on cnts (cost=0.00..14.00 rows=700 width=4) (actual
> time=0.005..148.464 rows=100 loops=1)
> -> Index Scan using alog_uid_idx on alog (cost=0.00..301.02 rows=1
> width=4) (actual time=2676.959..2676.959 rows=0 loops=100)
> Index Cond: ((alog.uid)::integer = (cnts.uid)::integer)
> Filter: ((alog.pid = 555949::numeric) AND (alog.odp = 1::numeric))
> Total runtime: 267844.942 ms
> (6 rows)
>
> The real running times are about 10 times more than the estimates. Is
> that normal?
>
> --
> Miernik
> http://miernik.name/
>
>
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