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

@@ -92,6 +92,8 @@
 #include "distributed/shard_pruning.h"
 #include "distributed/version_compat.h"
 
+bool EnableOuterJoinsWithPseudoconstantQualsPrePG17 = false;
+
 /*
  * RecursivePlanningContext is used to recursively plan subqueries
  * and CTEs, pull results to the coordinator, and push it back into
@@ -483,7 +485,7 @@ ShouldRecursivelyPlanOuterJoins(Query *query, RecursivePlanningContext *context)
 	bool hasOuterJoin =
 		context->plannerRestrictionContext->joinRestrictionContext->hasOuterJoin;
 
-	if (!hasOuterJoin)
+	if (!EnableOuterJoinsWithPseudoconstantQualsPrePG17 && !hasOuterJoin)
 	{
 		/*
 		 * PG15 commit d1ef5631e620f9a5b6480a32bb70124c857af4f1

src/backend/distributed/shared_library_init.c

@@ -1404,6 +1404,23 @@ 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_repartition_joins",
 		gettext_noop("Allows Citus to repartition data between nodes."),

src/include/distributed/recursive_planning.h

@@ -20,6 +20,7 @@
 #include "distributed/log_utils.h"
 #include "distributed/relation_restriction_equivalence.h"
 
+extern bool EnableOuterJoinsWithPseudoconstantQualsPrePG17;
 typedef struct RecursivePlanningContextInternal RecursivePlanningContext;
 
 typedef struct RangeTblEntryIndex