Re: too low cost of Bitmap index scan

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: too low cost of Bitmap index scan
Date: 2016-12-20 23:01:33
Message-ID: 6271.1482274893@sss.pgh.pa.us
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Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> I am trying to fix slow query on PostgreSQL 9.5.4.
> The data are almost in RAM

If it's all in RAM, you'd likely be well-served to lower random_page_cost.
It looks to me like the planner is estimating pretty accurately how many
heap fetches will be eliminated by using the extra index; where it's off
seems to be in the cost of those heap fetches relative to the index work.

regards, tom lane

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