Re: Hash join in 8.3

Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> But I'm curious if you turn off mergejoin whether you can get a Nested Loop
> plan and what cost 8.3 gives it. It looks to me like 8.3 came up with a higher
> cost for Nested Loop than 8.1.9 (I think 8.1.10 came out with some planner
> fixes btw) and so it's deciding these other plans are better. And they might
> have been better for the imaginary scenario that the planner thinks is going
> on.

Actually, now that I think about it, 8.3 should be *more* likely than
8.1 to choose a nestloop-with-inner-indexscan plan. 8.1 didn't have the
changes to allow a discount for repeated inner indexscans.

I'm wondering if

(a) the 8.1 installation being compared to had some planner cost
parameter changes that were not copied into the 8.3 installation; or

(b) the only reason 8.1 likes the nestloop plan is that it has no
statistics on the test tables, whereas 8.3 does have stats because
of autovacuum being on by default.

regards, tom lane