Add GUC for queries with outer joins and pseudoconstant quals (#8163)

Users can turn on this GUC at their own risk.

src/backend/distributed/planner/recursive_planning.c

@@ -97,6 +97,7 @@
 #include "distributed/version_compat.h"
 
 bool EnableRecurringOuterJoinPushdown = true;
+bool EnableOuterJoinsWithPseudoconstantQualsPrePG17 = false;
 
 /*
  * RecursivePlanningContext is used to recursively plan subqueries
@@ -509,7 +510,7 @@ ShouldRecursivelyPlanOuterJoins(Query *query, RecursivePlanningContext *context)
 	bool hasOuterJoin =
 		context->plannerRestrictionContext->joinRestrictionContext->hasOuterJoin;
 #if PG_VERSION_NUM < PG_VERSION_17
-	if (!hasOuterJoin)
+	if (!EnableOuterJoinsWithPseudoconstantQualsPrePG17 && !hasOuterJoin)
 	{
 		/*
 		 * PG15 commit d1ef5631e620f9a5b6480a32bb70124c857af4f1

src/backend/distributed/shared_library_init.c

@@ -1480,6 +1480,23 @@ RegisterCitusConfigVariables(void)
 		GUC_NO_SHOW_ALL | GUC_NOT_IN_SAMPLE,
 		NULL, NULL, NULL);
 
+	DefineCustomBoolVariable(
+		"citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17",
+		gettext_noop("Enables running distributed queries with outer joins "
+					 "and pseudoconstant quals pre PG17."),
+		gettext_noop("Set to false by default. If set to true, enables "
+					 "running distributed queries with outer joins and  "
+					 "pseudoconstant quals, at user's own risk, because "
+					 "pre PG17, Citus doesn't have access to "
+					 "set_join_pathlist_hook, which doesn't guarantee correct"
+					 "query results. Note that in PG17+, this GUC has no effect"
+					 "and the user can run such queries"),
+		&EnableOuterJoinsWithPseudoconstantQualsPrePG17,
+		false,
+		PGC_USERSET,
+		GUC_NO_SHOW_ALL | GUC_NOT_IN_SAMPLE,
+		NULL, NULL, NULL);
+
 	DefineCustomBoolVariable(
 		"citus.enable_recurring_outer_join_pushdown",
 		gettext_noop("Enables outer join pushdown for recurring relations."),

src/include/distributed/recursive_planning.h

@@ -22,6 +22,7 @@
 #include "distributed/relation_restriction_equivalence.h"
 
 extern bool EnableRecurringOuterJoinPushdown;
+extern bool EnableOuterJoinsWithPseudoconstantQualsPrePG17;
 typedef struct RecursivePlanningContextInternal RecursivePlanningContext;
 
 typedef struct RangeTblEntryIndex

src/test/regress/expected/pg17.out

@@ -422,6 +422,19 @@ where (exists (select  * from t4)) order by 1, 2, 3;
       |                          |    1 | Tue Mar 26 17:36:53 2024
 (2 rows)
 
+SET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17 TO true;
+-- wrong result pre-pg17
+select * from
+  (t0 full outer join t3
+    on (t0.c3 = t3.c26 ))
+where (exists (select  * from t4)) order by 1, 2, 3;
+ vkey |            c3            | vkey |           c26
+---------------------------------------------------------------------
+   13 | Wed Oct 23 15:34:50 2019 |      |
+      |                          |    1 | Tue Mar 26 17:36:53 2024
+(2 rows)
+
+RESET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17;
 -- issue https://github.com/citusdata/citus/issues/7696
 create table t1 ( vkey int4 );
 create table t2 ( vkey int4 );
@@ -456,6 +469,16 @@ where not((85) in (select 1 from t2));
     5 |
 (1 row)
 
+SET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17 TO true;
+-- wrong result pre-pg17
+select * from (t2 full outer join t1 on(t2.vkey = t1.vkey ))
+where not((85) in (select 1 from t2));
+ vkey | vkey
+---------------------------------------------------------------------
+    5 |
+(1 row)
+
+RESET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17;
 -- issue https://github.com/citusdata/citus/issues/7698
 create table t5 ( vkey int4, c10 int4 );
 create table t6 ( vkey int4 );
@@ -489,6 +512,18 @@ where exists (select * from t6);
     1
 (1 row)
 
+SET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17 TO true;
+-- wrong result pre-pg17
+select t6.vkey
+from (t5 right outer join t6
+    on (t5.c10 = t6.vkey))
+where exists (select * from t6);
+ vkey
+---------------------------------------------------------------------
+    1
+(1 row)
+
+RESET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17;
 -- issue https://github.com/citusdata/citus/issues/7119
 -- this test was removed in
 -- https://github.com/citusdata/citus/commit/a5ce601c0

src/test/regress/expected/pg17_0.out

@@ -374,6 +374,20 @@ where (exists (select  * from t4)) order by 1, 2, 3;
 ERROR:  Distributed queries with outer joins and pseudoconstant quals are not supported in PG15 and PG16.
 DETAIL:  PG15 and PG16 disallow replacing joins with scans when the query has pseudoconstant quals
 HINT:  Consider upgrading your PG version to PG17+
+SET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17 TO true;
+-- wrong result pre-pg17
+select * from
+  (t0 full outer join t3
+    on (t0.c3 = t3.c26 ))
+where (exists (select  * from t4)) order by 1, 2, 3;
+ vkey |            c3            | vkey |           c26
+---------------------------------------------------------------------
+   13 | Wed Oct 23 15:34:50 2019 |      |
+      |                          |    1 | Tue Mar 26 17:36:53 2024
+      |                          |    1 | Tue Mar 26 17:36:53 2024
+(3 rows)
+
+RESET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17;
 -- issue https://github.com/citusdata/citus/issues/7696
 create table t1 ( vkey int4 );
 create table t2 ( vkey int4 );
@@ -406,6 +420,17 @@ where not((85) in (select 1 from t2));
 ERROR:  Distributed queries with outer joins and pseudoconstant quals are not supported in PG15 and PG16.
 DETAIL:  PG15 and PG16 disallow replacing joins with scans when the query has pseudoconstant quals
 HINT:  Consider upgrading your PG version to PG17+
+SET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17 TO true;
+-- wrong result pre-pg17
+select * from (t2 full outer join t1 on(t2.vkey = t1.vkey ))
+where not((85) in (select 1 from t2));
+ vkey | vkey
+---------------------------------------------------------------------
+    5 |
+    5 |
+(2 rows)
+
+RESET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17;
 -- issue https://github.com/citusdata/citus/issues/7698
 create table t5 ( vkey int4, c10 int4 );
 create table t6 ( vkey int4 );
@@ -437,6 +462,19 @@ where exists (select * from t6);
 ERROR:  Distributed queries with outer joins and pseudoconstant quals are not supported in PG15 and PG16.
 DETAIL:  PG15 and PG16 disallow replacing joins with scans when the query has pseudoconstant quals
 HINT:  Consider upgrading your PG version to PG17+
+SET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17 TO true;
+-- wrong result pre-pg17
+select t6.vkey
+from (t5 right outer join t6
+    on (t5.c10 = t6.vkey))
+where exists (select * from t6);
+ vkey
+---------------------------------------------------------------------
+    1
+    1
+(2 rows)
+
+RESET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17;
 -- issue https://github.com/citusdata/citus/issues/7119
 -- this test was removed in
 -- https://github.com/citusdata/citus/commit/a5ce601c0

src/test/regress/sql/pg17.sql

@@ -220,6 +220,16 @@ select * from
     on (t0.c3 = t3.c26 ))
 where (exists (select  * from t4)) order by 1, 2, 3;
 
+SET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17 TO true;
+
+-- wrong result pre-pg17
+select * from
+  (t0 full outer join t3
+    on (t0.c3 = t3.c26 ))
+where (exists (select  * from t4)) order by 1, 2, 3;
+
+RESET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17;
+
 -- issue https://github.com/citusdata/citus/issues/7696
 create table t1 ( vkey int4 );
 create table t2 ( vkey int4 );
@@ -234,6 +244,14 @@ SELECT create_reference_table('t2');
 select * from (t2 full outer join t1 on(t2.vkey = t1.vkey ))
 where not((85) in (select 1 from t2));
 
+SET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17 TO true;
+
+-- wrong result pre-pg17
+select * from (t2 full outer join t1 on(t2.vkey = t1.vkey ))
+where not((85) in (select 1 from t2));
+
+RESET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17;
+
 -- issue https://github.com/citusdata/citus/issues/7698
 create table t5 ( vkey int4, c10 int4 );
 create table t6 ( vkey int4 );
@@ -252,6 +270,16 @@ from (t5 right outer join t6
     on (t5.c10 = t6.vkey))
 where exists (select * from t6);
 
+SET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17 TO true;
+
+-- wrong result pre-pg17
+select t6.vkey
+from (t5 right outer join t6
+    on (t5.c10 = t6.vkey))
+where exists (select * from t6);
+
+RESET citus.enable_outer_joins_with_pseudoconstant_quals_pre_pg17;
+
 -- issue https://github.com/citusdata/citus/issues/7119
 -- this test was removed in
 -- https://github.com/citusdata/citus/commit/a5ce601c0