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Fix tuple_fraction calculation in generate_orderedappend_paths()

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6b94e7a6da adjusted generate_orderedappend_paths() to consider fractional
paths.  However, it didn't manage to interpret the tuple_fraction value
correctly.  According to the header comment of grouping_planner(), the
tuple_fraction >= 1 specifies the absolute number of expected tuples.  That
number must be divided by the expected total number of tuples to get the
actual fraction.

Even though this is a bug fix, we don't backpatch it.  The risks of the side
effects of plan changes on stable branches are too high.

Reported-by: Andrei Lepikhov <lepihov@gmail.com>
Discussion: https://postgr.es/m/3ca271fa-ca5c-458c-8934-eb148622b270%40gmail.com
Author: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
This commit is contained in:
Alexander Korotkov
2025-05-18 23:49:50 +03:00
parent 12eee85e51
commit 3d3a81fc24
3 changed files with 32 additions and 1 deletions
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12
src/backend/optimizer/path/allpaths.c
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@@ -1891,7 +1891,17 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel,
*/
if (root->tuple_fraction > 0)
{
double path_fraction = (1.0 / root->tuple_fraction);
double path_fraction = root->tuple_fraction;
/*
* Merge Append considers only live children relations. Dummy
* relations must be filtered out before.
*/
Assert(childrel->rows > 0);
/* Convert absolute limit to a path fraction */
if (path_fraction >= 1.0)
path_fraction /= childrel->rows;
cheapest_fractional =
get_cheapest_fractional_path_for_pathkeys(childrel->pathlist,