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A little further tweaking of the range-query selectivity logic:

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to avoid undue sensitivity to roundoff error, believe that a zero
or slightly negative range estimate should represent a small
positive selectivity, rather than falling back on a generic default
estimate.
This commit is contained in:
Tom Lane 2000-03-23 23:35:47 +00:00
parent 6d79d6027c
commit 7177bbac29
1 changed files with 25 additions and 13 deletions
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38
src/backend/optimizer/path/clausesel.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/clausesel.c,v 1.32 2000/03/23 00:58:36 tgl Exp $
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/clausesel.c,v 1.33 2000/03/23 23:35:47 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -186,20 +186,32 @@ clauselist_selectivity(Query *root,
/* Successfully matched a pair of range clauses */
Selectivity s2 = rqlist->hibound + rqlist->lobound - 1.0;
if (s2 > 0.0)
/*
* A zero or slightly negative s2 should be converted into a
* small positive value; we probably are dealing with a very
* tight range and got a bogus result due to roundoff errors.
* However, if s2 is very negative, then we probably have
* default selectivity estimates on one or both sides of the
* range. In that case, insert a not-so-wildly-optimistic
* default estimate.
*/
if (s2 <= 0.0)
{
/* All our hard work has paid off! */
s1 *= s2;
}
else
{
/* One or both is probably a default estimate,
* so supply a default estimate for the selectivity
* of the range query. We rather optimistically assume
* that the range is tight...
*/
s1 *= 0.01;
if (s2 < -0.01)
{
/* No data available --- use a default estimate that
* is small, but not real small.
*/
s2 = 0.01;
}
else
{
/* It's just roundoff error; use a small positive value */
s2 = 1.0e-10;
}
}
/* Merge in the selectivity of the pair of clauses */
s1 *= s2;
}
else
{