Re: [PERFORM] bitmap scan issues 8.1 devel

> "Merlin Moncure" <merlin.moncure@rcsonline.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.

You nailed it...I hard coded the limit and everything was cool.  In
fact, the same problem contributes to the fact that I've had to run
seqscan=false on all my production systems.  It seemed the planner would
randomly seqscan the table...now I know why.

> (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?)

I absolutely support this :)  In normal usage, the supplied limit is
quite small, say 100 or less.  Anyways, planner issues aside,
parameterizing the limit is an elegant way to read records off a table
when you don't know how many you are going to read in advance...I make
heavy use of it :(.

Merlin