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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
View File

@ -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,

18
src/test/regress/expected/partition_join.out
View File

@ -5260,6 +5260,24 @@ SELECT x.id, y.id FROM fract_t x LEFT JOIN fract_t y USING (id) ORDER BY x.id DE
Index Cond: (id = x_2.id)
(11 rows)
EXPLAIN (COSTS OFF) -- Should use NestLoop with parameterised inner scan
SELECT x.id, y.id FROM fract_t x LEFT JOIN fract_t y USING (id)
ORDER BY x.id DESC LIMIT 2;
QUERY PLAN
--------------------------------------------------------------------------------
Limit
-> Merge Append
Sort Key: x.id DESC
-> Nested Loop Left Join
-> Index Only Scan Backward using fract_t0_pkey on fract_t0 x_1
-> Index Only Scan using fract_t0_pkey on fract_t0 y_1
Index Cond: (id = x_1.id)
-> Nested Loop Left Join
-> Index Only Scan Backward using fract_t1_pkey on fract_t1 x_2
-> Index Only Scan using fract_t1_pkey on fract_t1 y_2
Index Cond: (id = x_2.id)
(11 rows)
--
-- Test Append's fractional paths
--

3
src/test/regress/sql/partition_join.sql
View File

@ -1224,6 +1224,9 @@ SELECT x.id, y.id FROM fract_t x LEFT JOIN fract_t y USING (id) ORDER BY x.id AS
EXPLAIN (COSTS OFF)
SELECT x.id, y.id FROM fract_t x LEFT JOIN fract_t y USING (id) ORDER BY x.id DESC LIMIT 10;
EXPLAIN (COSTS OFF) -- Should use NestLoop with parameterised inner scan
SELECT x.id, y.id FROM fract_t x LEFT JOIN fract_t y USING (id)
ORDER BY x.id DESC LIMIT 2;
--
-- Test Append's fractional paths