Fix semijoin unique-ification for child relations

For a child relation, we should not assume that its parent's
unique-ified relation (or unique-ified path in v18) always exists.  In
cases where all RHS columns that need to be unique-ified are equated
to constants, the unique-ified relation/path for the parent table is
not built, as there are no columns left to unique-ify.  Failing to
account for this can result in a SIGSEGV crash during planning.

This patch checks whether the parent's unique-ified relation or path
exists and skips unique-ification of the child relation if it does
not.

Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAMbWs49MOdLW2c+qbLHHBt8VBu=4ONpM91D19=AWeW93eFUF6A@mail.gmail.com
Backpatch-through: 18

src/backend/optimizer/plan/planner.c

@@ -8307,6 +8307,15 @@ create_unique_paths(PlannerInfo *root, RelOptInfo *rel, SpecialJoinInfo *sjinfo)
 	if (!(sjinfo->semi_can_btree || sjinfo->semi_can_hash))
 		return NULL;
 
+	/*
+	 * Punt if this is a child relation and we failed to build a unique-ified
+	 * relation for its parent.  This can happen if all the RHS columns were
+	 * found to be equated to constants when unique-ifying the parent table,
+	 * leaving no columns to unique-ify.
+	 */
+	if (IS_OTHER_REL(rel) && rel->top_parent->unique_rel == NULL)
+		return NULL;
+
 	/*
 	 * When called during GEQO join planning, we are in a short-lived memory
 	 * context.  We must make sure that the unique rel and any subsidiary data

src/test/regress/expected/join.out

@@ -6580,6 +6580,66 @@ where exists (select 1 from t t4
  1
 (1 row)
 
+rollback;
+-- check handling of semijoins if all RHS columns are equated to constants: we
+-- should suppress unique-ification in this case.
+begin;
+create temp table t (a int, b int);
+insert into t values (1, 2);
+explain (costs off)
+select * from t t1, t t2 where exists
+  (select 1 from t t3 where t1.a = t3.a and t2.b = t3.b and t3.a = 1 and t3.b = 2);
+                 QUERY PLAN                  
+---------------------------------------------
+ Nested Loop Semi Join
+   ->  Nested Loop
+         ->  Seq Scan on t t1
+               Filter: (a = 1)
+         ->  Materialize
+               ->  Seq Scan on t t2
+                     Filter: (b = 2)
+   ->  Materialize
+         ->  Seq Scan on t t3
+               Filter: ((a = 1) AND (b = 2))
+(10 rows)
+
+select * from t t1, t t2 where exists
+  (select 1 from t t3 where t1.a = t3.a and t2.b = t3.b and t3.a = 1 and t3.b = 2);
+ a | b | a | b 
+---+---+---+---
+ 1 | 2 | 1 | 2
+(1 row)
+
+rollback;
+-- check handling of semijoin unique-ification for child relations if all RHS
+-- columns are equated to constants.
+begin;
+create temp table p (a int, b int) partition by range (a);
+create temp table p1 partition of p for values from (0) to (10);
+create temp table p2 partition of p for values from (10) to (20);
+insert into p values (1, 2);
+insert into p values (10, 20);
+set enable_partitionwise_join to on;
+explain (costs off)
+select * from p t1 where exists
+  (select 1 from p t2 where t1.a = t2.a and t1.a = 1);
+          QUERY PLAN           
+-------------------------------
+ Nested Loop Semi Join
+   ->  Seq Scan on p1 t1
+         Filter: (a = 1)
+   ->  Materialize
+         ->  Seq Scan on p1 t2
+               Filter: (a = 1)
+(6 rows)
+
+select * from p t1 where exists
+  (select 1 from p t2 where t1.a = t2.a and t1.a = 1);
+ a | b 
+---+---
+ 1 | 2
+(1 row)
+
 rollback;
 -- test cases where we can remove a join, but not a PHV computed at it
 begin;

src/test/regress/sql/join.sql

@@ -2453,6 +2453,43 @@ where exists (select 1 from t t4
 
 rollback;
 
+-- check handling of semijoins if all RHS columns are equated to constants: we
+-- should suppress unique-ification in this case.
+begin;
+
+create temp table t (a int, b int);
+insert into t values (1, 2);
+
+explain (costs off)
+select * from t t1, t t2 where exists
+  (select 1 from t t3 where t1.a = t3.a and t2.b = t3.b and t3.a = 1 and t3.b = 2);
+
+select * from t t1, t t2 where exists
+  (select 1 from t t3 where t1.a = t3.a and t2.b = t3.b and t3.a = 1 and t3.b = 2);
+
+rollback;
+
+-- check handling of semijoin unique-ification for child relations if all RHS
+-- columns are equated to constants.
+begin;
+
+create temp table p (a int, b int) partition by range (a);
+create temp table p1 partition of p for values from (0) to (10);
+create temp table p2 partition of p for values from (10) to (20);
+insert into p values (1, 2);
+insert into p values (10, 20);
+
+set enable_partitionwise_join to on;
+
+explain (costs off)
+select * from p t1 where exists
+  (select 1 from p t2 where t1.a = t2.a and t1.a = 1);
+
+select * from p t1 where exists
+  (select 1 from p t2 where t1.a = t2.a and t1.a = 1);
+
+rollback;
+
 -- test cases where we can remove a join, but not a PHV computed at it
 begin;