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Re: [HACKERS] bitmap scan issues 8.1 devel

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Merlin Moncure" <merlin(dot)moncure(at)rcsonline(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [HACKERS] bitmap scan issues 8.1 devel
Date: 2005-08-17 21:54:36
Message-ID: 17374.1124315676@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
"Merlin Moncure" <merlin(dot)moncure(at)rcsonline(dot)com> writes:
> Doing some testing on upcoming 8.1 devel and am having serious issues
> with new bitmap index scan feature.  It is easy to work around (just
> disable it) but IMO the planner is using it when a regular index scan
> should be strongly favored.

I think blaming the bitmap code is the wrong response.  What I see in
your example is that the planner doesn't know what the LIMIT value is,
and accordingly is favoring a plan that isn't going to get blown out of
the water if the LIMIT is large.  I'd suggest not parameterizing the
LIMIT.

(But hmm ... I wonder if we could use estimate_expression_value for
LIMIT items, instead of handling only simple Consts as the code does
now?)

			regards, tom lane

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