Re: index scan cost

Jeff Frost <jeff@frostconsultingllc.com> writes:
> I have two postgresql servers.  One runs 8.3.1, the other 8.3.3.  On the 8.3.1
> machine, the index scans are being planned extremely low cost:

>   Index Scan using ix_email_entity_thread on email_entity  (cost=0.00..4.59
> rows=1 width=1031) (actual time=0.095..0.120 rows=4 loops=1)
>     Index Cond: (email_thread = 375629157)

>   Index Scan using ix_email_entity_thread on email_entity  (cost=0.00..2218.61
> rows=1151 width=931) (actual time=0.094..0.111 rows=4 loops=1)
>     Index Cond: (email_thread = 375629157)

This isn't a "cost" problem, this is a "stats" problem.  Why does the
second server think 1151 rows will be returned?  Try comparing the
pg_stats entries for the email_thread column on both servers ... seems
like they must be significantly different.

            regards, tom lane