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Make estimation of mergejoin scan selectivities more robust, per recent

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example from RaÇl GutiÅrrez.
This commit is contained in:
Tom Lane
2003-01-22 20:17:07 +00:00
parent 1f74045675
commit c0f8da118b
2 changed files with 43 additions and 13 deletions
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20
src/backend/optimizer/path/costsize.c
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@@ -42,7 +42,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.90 2002/09/04 20:31:20 momjian Exp $
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.90.2.1 2003/01/22 20:17:06 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -645,8 +645,22 @@ cost_mergejoin(Path *path, Query *root,
innerscansel = firstclause->left_mergescansel;
}
outer_rows = outer_path->parent->rows * outerscansel;
inner_rows = inner_path->parent->rows * innerscansel;
/* convert selectivity to row count; must scan at least one row */
outer_rows = ceil(outer_path->parent->rows * outerscansel);
if (outer_rows < 1)
outer_rows = 1;
inner_rows = ceil(inner_path->parent->rows * innerscansel);
if (inner_rows < 1)
inner_rows = 1;
/*
* Readjust scan selectivities to account for above rounding. This is
* normally an insignificant effect, but when there are only a few rows
* in the inputs, failing to do this makes for a large percentage error.
*/
outerscansel = outer_rows / outer_path->parent->rows;
innerscansel = inner_rows / inner_path->parent->rows;
/* cost of source data */