Disallow partitionwise join when collations don't match

If the collation of any join key column doesn’t match the collation of
the corresponding partition key, partitionwise joins can yield incorrect
results. For example, rows that would match under the join key collation
might be located in different partitions due to the partitioning
collation. In such cases, a partitionwise join would yield different
results from a non-partitionwise join, so disallow it in such cases.

Reported-by: Tender Wang <tndrwang@gmail.com>
Author: Jian He <jian.universality@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Discussion: https://postgr.es/m/CAHewXNno_HKiQ6PqyLYfuqDtwp7KKHZiH1J7Pqyz0nr+PS2Dwg@mail.gmail.com
Backpatch-through: 12

3 files changed, 144 insertions, 0 deletions

diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c
index 836bdaac069..a496f56b4d4 100644
--- a/src/backend/optimizer/util/relnode.c
+++ b/src/backend/optimizer/util/relnode.c
@@ -1807,6 +1807,10 @@ have_partkey_equi_join(RelOptInfo *joinrel,
 		if (ipk1 != ipk2)
 			continue;
 
+		/* Reject if the partition key collation differs from the clause's. */
+		if (rel1->part_scheme->partcollation[ipk1] != opexpr->inputcollid)
+			return false;
+
 		/*
 		 * The clause allows partitionwise join only if it uses the same
 		 * operator family as that specified by the partition key.
diff --git a/src/test/regress/expected/collate.icu.utf8.out b/src/test/regress/expected/collate.icu.utf8.out
index 164fd20807b..02a946c2ef6 100644
--- a/src/test/regress/expected/collate.icu.utf8.out
+++ b/src/test/regress/expected/collate.icu.utf8.out
@@ -2036,6 +2036,124 @@ SELECT c collate "C", count(c) FROM pagg_tab3 GROUP BY c collate "C" ORDER BY 1;
  b |     5
 (4 rows)
 
+-- Partitionwise join should not be allowed too when the collation used by the
+-- join keys doesn't match the partition key collation.
+SET enable_partitionwise_join TO false;
+EXPLAIN (COSTS OFF)
+SELECT t1.c, count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c GROUP BY 1 ORDER BY t1.c COLLATE "C";
+                         QUERY PLAN                          
+-------------------------------------------------------------
+ Sort
+   Sort Key: t1.c COLLATE "C"
+   ->  HashAggregate
+         Group Key: t1.c
+         ->  Hash Join
+               Hash Cond: (t1.c = t2.c)
+               ->  Append
+                     ->  Seq Scan on pagg_tab3_p2 t1_1
+                     ->  Seq Scan on pagg_tab3_p1 t1_2
+               ->  Hash
+                     ->  Append
+                           ->  Seq Scan on pagg_tab3_p2 t2_1
+                           ->  Seq Scan on pagg_tab3_p1 t2_2
+(13 rows)
+
+SELECT t1.c, count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c GROUP BY 1 ORDER BY t1.c COLLATE "C";
+ c | count 
+---+-------
+ A |   100
+ B |   100
+(2 rows)
+
+SET enable_partitionwise_join TO true;
+EXPLAIN (COSTS OFF)
+SELECT t1.c, count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c GROUP BY 1 ORDER BY t1.c COLLATE "C";
+                         QUERY PLAN                          
+-------------------------------------------------------------
+ Sort
+   Sort Key: t1.c COLLATE "C"
+   ->  HashAggregate
+         Group Key: t1.c
+         ->  Hash Join
+               Hash Cond: (t1.c = t2.c)
+               ->  Append
+                     ->  Seq Scan on pagg_tab3_p2 t1_1
+                     ->  Seq Scan on pagg_tab3_p1 t1_2
+               ->  Hash
+                     ->  Append
+                           ->  Seq Scan on pagg_tab3_p2 t2_1
+                           ->  Seq Scan on pagg_tab3_p1 t2_2
+(13 rows)
+
+SELECT t1.c, count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c GROUP BY 1 ORDER BY t1.c COLLATE "C";
+ c | count 
+---+-------
+ A |   100
+ B |   100
+(2 rows)
+
+-- OK when the join clause uses the same collation as the partition key.
+EXPLAIN (COSTS OFF)
+SELECT t1.c COLLATE "C", count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c COLLATE "C" GROUP BY t1.c COLLATE "C" ORDER BY t1.c COLLATE "C";
+                            QUERY PLAN                            
+------------------------------------------------------------------
+ Sort
+   Sort Key: ((t1.c)::text) COLLATE "C"
+   ->  Append
+         ->  HashAggregate
+               Group Key: (t1.c)::text
+               ->  Hash Join
+                     Hash Cond: ((t1.c)::text = (t2.c)::text)
+                     ->  Seq Scan on pagg_tab3_p2 t1
+                     ->  Hash
+                           ->  Seq Scan on pagg_tab3_p2 t2
+         ->  HashAggregate
+               Group Key: (t1_1.c)::text
+               ->  Hash Join
+                     Hash Cond: ((t1_1.c)::text = (t2_1.c)::text)
+                     ->  Seq Scan on pagg_tab3_p1 t1_1
+                     ->  Hash
+                           ->  Seq Scan on pagg_tab3_p1 t2_1
+(17 rows)
+
+SELECT t1.c COLLATE "C", count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c COLLATE "C" GROUP BY t1.c COLLATE "C" ORDER BY t1.c COLLATE "C";
+ c | count 
+---+-------
+ A |    25
+ B |    25
+ a |    25
+ b |    25
+(4 rows)
+
+SET enable_partitionwise_join TO false;
+EXPLAIN (COSTS OFF)
+SELECT t1.c COLLATE "C", count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c COLLATE "C" GROUP BY t1.c COLLATE "C" ORDER BY t1.c COLLATE "C";
+                         QUERY PLAN                          
+-------------------------------------------------------------
+ Sort
+   Sort Key: ((t1.c)::text) COLLATE "C"
+   ->  HashAggregate
+         Group Key: (t1.c)::text
+         ->  Hash Join
+               Hash Cond: ((t1.c)::text = (t2.c)::text)
+               ->  Append
+                     ->  Seq Scan on pagg_tab3_p2 t1_1
+                     ->  Seq Scan on pagg_tab3_p1 t1_2
+               ->  Hash
+                     ->  Append
+                           ->  Seq Scan on pagg_tab3_p2 t2_1
+                           ->  Seq Scan on pagg_tab3_p1 t2_2
+(13 rows)
+
+SELECT t1.c COLLATE "C", count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c COLLATE "C" GROUP BY t1.c COLLATE "C" ORDER BY t1.c COLLATE "C";
+ c | count 
+---+-------
+ A |    25
+ B |    25
+ a |    25
+ b |    25
+(4 rows)
+
 DROP TABLE pagg_tab3;
 RESET enable_partitionwise_aggregate;
 RESET max_parallel_workers_per_gather;
diff --git a/src/test/regress/sql/collate.icu.utf8.sql b/src/test/regress/sql/collate.icu.utf8.sql
index 12fcfdb5a79..b093e1d1670 100644
--- a/src/test/regress/sql/collate.icu.utf8.sql
+++ b/src/test/regress/sql/collate.icu.utf8.sql
@@ -779,6 +779,28 @@ EXPLAIN (COSTS OFF)
 SELECT c collate "C", count(c) FROM pagg_tab3 GROUP BY c collate "C" ORDER BY 1;
 SELECT c collate "C", count(c) FROM pagg_tab3 GROUP BY c collate "C" ORDER BY 1;
 
+-- Partitionwise join should not be allowed too when the collation used by the
+-- join keys doesn't match the partition key collation.
+SET enable_partitionwise_join TO false;
+EXPLAIN (COSTS OFF)
+SELECT t1.c, count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c GROUP BY 1 ORDER BY t1.c COLLATE "C";
+SELECT t1.c, count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c GROUP BY 1 ORDER BY t1.c COLLATE "C";
+
+SET enable_partitionwise_join TO true;
+EXPLAIN (COSTS OFF)
+SELECT t1.c, count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c GROUP BY 1 ORDER BY t1.c COLLATE "C";
+SELECT t1.c, count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c GROUP BY 1 ORDER BY t1.c COLLATE "C";
+
+-- OK when the join clause uses the same collation as the partition key.
+EXPLAIN (COSTS OFF)
+SELECT t1.c COLLATE "C", count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c COLLATE "C" GROUP BY t1.c COLLATE "C" ORDER BY t1.c COLLATE "C";
+SELECT t1.c COLLATE "C", count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c COLLATE "C" GROUP BY t1.c COLLATE "C" ORDER BY t1.c COLLATE "C";
+
+SET enable_partitionwise_join TO false;
+EXPLAIN (COSTS OFF)
+SELECT t1.c COLLATE "C", count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c COLLATE "C" GROUP BY t1.c COLLATE "C" ORDER BY t1.c COLLATE "C";
+SELECT t1.c COLLATE "C", count(t2.c) FROM pagg_tab3 t1 JOIN pagg_tab3 t2 ON t1.c = t2.c COLLATE "C" GROUP BY t1.c COLLATE "C" ORDER BY t1.c COLLATE "C";
+
 DROP TABLE pagg_tab3;
 
 RESET enable_partitionwise_aggregate;