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Fix tuple_fraction calculation in generate_orderedappend_paths()

author Alexander Korotkov <akorotkov@postgresql.org>

2025年5月18日 20:49:50 +0000 (23:49 +0300)

committer Alexander Korotkov <akorotkov@postgresql.org>

2025年5月18日 20:49:50 +0000 (23:49 +0300)

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>

src/backend/optimizer/path/allpaths.c patch | blob | blame | history

src/test/regress/expected/partition_join.out patch | blob | blame | history

src/test/regress/sql/partition_join.sql patch | blob | blame | history

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c

index 905250b3325251e121f6b09a79c995ae77b1aea6..6cc6966b0600acb3fda3b8cafd1e2ec6c700fb7a 100644 (file)

--- a/src/backend/optimizer/path/allpaths.c

+++ b/src/backend/optimizer/path/allpaths.c

@@ -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,

diff --git a/src/test/regress/expected/partition_join.out b/src/test/regress/expected/partition_join.out

index 6101c8c7cf10841c19cf2573280ae952762d459e..d5368186caa9f0b97cd50d4b4a54151f402afa88 100644 (file)

--- a/src/test/regress/expected/partition_join.out

+++ b/src/test/regress/expected/partition_join.out

@@ -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

--

diff --git a/src/test/regress/sql/partition_join.sql b/src/test/regress/sql/partition_join.sql

index b76c545100126ff1554ae2b4f6d10139dd4d4011..30f15ee9acb5a5e3b8463e457e65891f23e32a57 100644 (file)

--- a/src/test/regress/sql/partition_join.sql

+++ b/src/test/regress/sql/partition_join.sql

@@ -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

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