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Cause planner to make use of average-column-width statistic that is now

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collected by ANALYZE.  Also, add some modest amount of intelligence to
guesses that are used for varlena columns in the absence of any ANALYZE
statistics.  The 'width' reported by EXPLAIN is finally something less
than totally bogus for varlena columns ... and, in consequence, hashjoin
estimating should be a little better ...
This commit is contained in:
Tom Lane
2001-05-09 00:35:09 +00:00
parent b66a0cca57
commit 6cda3ad8fe
4 changed files with 124 additions and 42 deletions
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9
doc/src/sgml/perform.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/perform.sgml,v 1.3 2001/03/24 23:03:26 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/perform.sgml,v 1.4 2001/05/09 00:35:09 tgl Exp $
-->
<chapter id="performance-tips">
@ -97,13 +97,6 @@ $Header: /cvsroot/pgsql/doc/src/sgml/perform.sgml,v 1.3 2001/03/24 23:03:26 pete
by the query (again, without considering the effects of LIMIT).
</para>
<para>
Average width is pretty bogus because the thing really doesn't have
any idea of the average length of variable-length columns. I'm thinking
about improving that in the future, but it may not be worth the trouble,
because the width isn't used for very much.
</para>
<para>
Here are some examples (using the regress test database after a
vacuum analyze, and almost-7.0 sources):