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Re: LIMIT confuses the planner

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: kouber(at)saparev(dot)com, pgsql-performance(at)postgresql(dot)org
Subject: Re: LIMIT confuses the planner
Date: 2009-02-23 15:09:49
Message-ID: 3562.1235401789@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> If you left seq_page_cost (which isn't mentioned here) at the default
> value but reduced random_page_cost to 0.1, then you have
> random_page_cost < seq_page_cost.  That's probably Bad.

... well, it's certainly going to push the planner to believe indexscans
are cheaper than sorts no matter what.

The previously noted rowcount estimation problem might be a bigger issue
in this particular case, but I agree this is a Bad Idea.

			regards, tom lane

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