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Re: [PERFORM] Slow query: bitmap scan troubles

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [PERFORM] Slow query: bitmap scan troubles
Date: 2013-01-07 18:03:37
Message-ID: CA+U5nMKbOGVfQXfJi5_vOUPEatF_V_+e_HX4P5R=tb9JSo2ceA@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
On 7 January 2013 17:35, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> That gives a formula of
>
>         cpu_operator_cost * log2(N) + cpu_operator_cost * 50 * (H+2)
>
> This would lead to the behavior depicted in the attached plot, wherein
> I've modified the comparison lines (historical, 9.2, and HEAD behaviors)
> to include the existing 100 * cpu_operator_cost startup cost charge in
> addition to the fudge factor we've been discussing so far.  The new
> proposed curve is a bit above the historical curve for indexes with
> 250-5000 tuples, but the value is still quite small there, so I'm not
> too worried about that.  The people who've been complaining about 9.2's
> behavior have indexes much larger than that.
>
> Thoughts?

Again, this depends on N and H, so thats good.

I think my retinas detached while reading your explanation, but I'm a
long way from coming up with a better or more principled one.

If we can describe this as a heuristic that appears to fit the
observed costs, we may keep the door open for something better a
little later.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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