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 <[EMAIL PROTECTED]> wrote: > Miernik <[EMAIL PROTECTED]> 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/ > > > -- > Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance >
