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Re: Selectivity estimation for equality and range queries

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Peter Eisentraut <peter_e(at)gmx(dot)net>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Selectivity estimation for equality and range queries
Date: 2007-12-28 17:29:26
Message-ID: 4454.1198862966@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Peter Eisentraut <peter_e(at)gmx(dot)net> writes:
> I have been observing a case where the row count estimation for LIKE 'foo' is
> (much) higher than for LIKE 'foo%', the rest of the query being the same.  
> This is a special case of the estimation for equality being higher than for a
> range query that includes the value used in the equality.

Not really --- LIKE estimation is only weakly related to range
estimation.

Relevant questions here include exactly which PG version is in use and
what's the database encoding/locale.  If it's not C locale, the fixes
I made in selfuncs.c during November might be relevant.

			regards, tom lane

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