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Update comments about clause selectivity estimation.

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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21
src/backend/optimizer/path/allpaths.c
View File

@ -7,7 +7,7 @@
*
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/allpaths.c,v 1.51 1999/07/24 23:21:08 tgl Exp $
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/allpaths.c,v 1.52 1999/07/30 22:34:17 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -59,6 +59,9 @@ make_one_rel(Query *root, List *rels)
if (levels_needed <= 0)
return NULL;
/*
* Generate access paths for the base rels.
*/
set_base_rel_pathlist(root, rels);
if (levels_needed <= 1)
@ -73,8 +76,10 @@ make_one_rel(Query *root, List *rels)
{
/*
* This means that joins or sorts are required. set selectivities
* of clauses that have not been set by an index.
* This means that joins or sorts are required. Set selectivities
* of any clauses not yet set. (I think that this is redundant;
* set_base_rel_pathlist should have set them all already. But
* a scan to check that they are all set doesn't cost much...)
*/
set_rest_relselec(root, rels);
@ -131,11 +136,15 @@ set_base_rel_pathlist(Query *root, List *rels)
set_cheapest(rel, rel->pathlist);
/*
* if there is a qualification of sequential scan the selec. value
* is not set -- so set it explicitly -- Sunita
/* Set the selectivity estimates for any restriction clauses that
* didn't get set as a byproduct of index-path selectivity estimation
* (see create_index_path()).
*/
set_rest_selec(root, rel->restrictinfo);
/* Calculate the estimated size (post-restrictions) and tuple width
* for this base rel. This uses the restriction clause selectivities.
*/
rel->size = compute_rel_size(rel);
rel->width = compute_rel_width(rel);
}

20
src/backend/optimizer/util/pathnode.c
View File

@ -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);