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Re: contsel and gist

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Ben <midfield(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: contsel and gist
Date: 2010-10-28 21:19:40
Message-ID: 26257.1288300780@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Ben <midfield(at)gmail(dot)com> writes:
> On Oct 28, 2010, at 10:50 AM, Tom Lane wrote:
>> However, having said that: the constant value of the stub contsel
>> function is intended to be small enough to encourage use of an
>> indexscan.  Maybe we just need to decrease it a bit more.  Have you
>> investigated what the cutover point is for your queries?

> i'd be happy to investigate this for you, but my guess is my dataset
is probably not a good example to use for setting the constant more
generally.  i'm joining an 8e10 table vs a 150K table, so the
selectivity fraction would probably have to drop by many orders of
magnitude.

I doubt it.

> that being said, i'll poke around and see if i can find the cutoff point.  is there an easy way to do this that doesn't involve recompiling postgres?

No, those are just hardwired constants.  If you wanted to avoid multiple
recompilations while experimenting, you could set up a custom GUC
variable for the functions to read...

			regards, tom lane

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