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Update comments about clause selectivity estimation.

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This commit is contained in:
Tom Lane
1999-07-30 22:34:19 +00:00
parent 44763a2b23
commit 30da344cb1
2 changed files with 27 additions and 14 deletions
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20
src/backend/optimizer/util/pathnode.c
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@ -7,7 +7,7 @@
*
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/optimizer/util/pathnode.c,v 1.51 1999/07/30 04:07:25 tgl Exp $
* $Header: /cvsroot/pgsql/src/backend/optimizer/util/pathnode.c,v 1.52 1999/07/30 22:34:19 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -412,14 +412,18 @@ create_index_path(Query *root,
/*
* Set selectivities of clauses used with index to the selectivity
* of this index, subdividing the selectivity equally over each of
* the clauses.
* the clauses. To the extent that index_selectivity() can make a
* better estimate of the joint selectivity of these clauses than
* the product of individual estimates from compute_clause_selec()
* would be, this should give us a more accurate estimate of the
* total selectivity of all the clauses.
*
* XXX Can this divide the selectivities in a better way?
*
* XXX In fact, why the heck are we doing this at all? We already
* set the cost for the indexpath, and it's far from obvious that
* the selectivity of the path should have any effect on estimates
* made for other contexts...
* XXX If there is more than one useful index for this rel, and the
* indexes can be used with different but overlapping groups of
* restriction clauses, we may end up with too optimistic an estimate,
* since set_clause_selectivities() will save the minimum of the
* per-clause selectivity estimated with each index. But that should
* be fairly unlikely for typical index usage.
*/
clausesel = pow(selec, 1.0 / (double) length(restriction_clauses));
set_clause_selectivities(restriction_clauses, clausesel);