From: Tom Lane <tgl@sss.pgh.pa.us>
To: Victor Yegorov <vyegorov@gmail.com>
cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Estimates on partial index
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Comments: In-reply-to Victor Yegorov <vyegorov@gmail.com>
	message dated "Thu, 18 Aug 2016 16:52:11 +0300"
Date: Thu, 18 Aug 2016 09:56:10 -0400
Message-ID: <766.1471528570@sss.pgh.pa.us>
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Victor Yegorov <vyegorov@gmail.com> writes:
> Settings:
>     random_page_cost     2.5      ¤
>     seq_page_cost        1        ¤

> Question is — why IndexScan over partial index is estimated less than
> BitmapHeap + BitmapIndex scan. And how can I tell Planner, that IndexScan
> over 1/3 of table is not a good thing — IndexScan is touching 10x more
> pages and in a typical situation those are cold.

In that case you've got random_page_cost too far down.  Values less than
the default of 4 are generally only appropriate if the bulk of your
database stays in RAM.

			regards, tom lane


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