Subject: Re: NOT LIKE much faster than LIKE?
From: Simon Riggs <simon@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Matteo Beccati <php@beccati.com>, Andrea Arcangeli <andrea@cpushare.com>,
	pgsql-performance@postgresql.org
In-Reply-To: <7924.1136950830@sss.pgh.pa.us>
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Date: Wed, 11 Jan 2006 09:07:45 +0000
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On Tue, 2006-01-10 at 22:40 -0500, Tom Lane wrote:
> Simon Riggs <simon@2ndquadrant.com> writes:
> > I meant use the same sampling approach as I was proposing for ANALYZE,
> > but do this at plan time for the query. That way we can apply the
> > function directly to the sampled rows and estimate selectivity. 
> 
> I think this is so unlikely to be a win as to not even be worth spending
> any time discussing.  The extra planning time across all queries will
> vastly outweigh the occasional improvement in plan choice for some
> queries.

Extra planning time would be bad, so clearly we wouldn't do this when we
already have relevant ANALYZE statistics. 

I would suggest we do this only when all of these are true
- when accessing more than one table, so the selectivity could effect a
join result
- when we have either no ANALYZE statistics, or ANALYZE statistics are
not relevant to estimating selectivity, e.g. LIKE 
- when access against the single table in question cannot find an index
to use from other RestrictInfo predicates

I imagined that this would also be controlled by a GUC, dynamic_sampling
which would be set to zero by default, and give a measure of sample size
to use. (Or just a bool enable_sampling = off (default)).

This is mentioned now because the plan under consideration in this
thread would be improved by this action. It also isn't a huge amount of
code to get it to work.

Best Regards, Simon Riggs