diff --git a/src/backend/distributed/planner/multi_logical_planner.c b/src/backend/distributed/planner/multi_logical_planner.c
index 51e1c48ed..db9aea8be 100644
--- a/src/backend/distributed/planner/multi_logical_planner.c
+++ b/src/backend/distributed/planner/multi_logical_planner.c
@@ -56,6 +56,21 @@ typedef struct QualifierWalkerContext
 } QualifierWalkerContext;
 
 
+/*
+ * RecurringTuplesType is used to distinguish different types of expressions
+ * that always produce the same set of tuples when a shard is queried. We make
+ * this distinction to produce relevant error messages when recurring tuples
+ * are used in a way that would give incorrect results.
+ */
+typedef enum RecurringTuplesType
+{
+	RECURRING_TUPLES_INVALID = 0,
+	RECURRING_TUPLES_REFERENCE_TABLE,
+	RECURRING_TUPLES_FUNCTION,
+	RECURRING_TUPLES_EMPTY_JOIN_TREE
+} RecurringTuplesType;
+
+
 /* Function pointer type definition for apply join rule functions */
 typedef MultiNode *(*RuleApplyFunction) (MultiNode *leftNode, MultiNode *rightNode,
 										 Var *partitionColumn, JoinType joinType,
@@ -90,9 +105,9 @@ static FieldSelect * CompositeFieldRecursive(Expr *expression, Query *query);
 static bool FullCompositeFieldList(List *compositeFieldList);
 static MultiNode * MultiPlanTree(Query *queryTree);
 static void ErrorIfQueryNotSupported(Query *queryTree);
-static bool HasUnsupportedReferenceTableJoin(
+static DeferredErrorMessage * DeferredErrorIfUnsupportedRecurringTuplesJoin(
 	PlannerRestrictionContext *plannerRestrictionContext);
-static bool ShouldRecurseForReferenceTableJoinChecks(RelOptInfo *relOptInfo);
+static bool ShouldRecurseForRecurringTuplesJoinChecks(RelOptInfo *relOptInfo);
 static bool HasUnsupportedJoinWalker(Node *node, void *context);
 static bool ErrorHintRequired(const char *errorHint, Query *queryTree);
 static DeferredErrorMessage * DeferErrorIfUnsupportedSubqueryRepartition(Query *
@@ -102,8 +117,10 @@ static bool HasOuterJoin(Query *queryTree);
 static bool HasOuterJoinWalker(Node *node, void *maxJoinLevel);
 static bool HasComplexJoinOrder(Query *queryTree);
 static bool HasComplexRangeTableType(Query *queryTree);
-static bool RelationInfoContainsReferenceTable(PlannerInfo *plannerInfo,
-											   RelOptInfo *relationInfo);
+static bool RelationInfoContainsRecurringTuples(PlannerInfo *plannerInfo,
+												RelOptInfo *relationInfo,
+												RecurringTuplesType *recurType);
+static bool HasRecurringTuples(Node *node, RecurringTuplesType *recurType);
 static void ValidateClauseList(List *clauseList);
 static void ValidateSubqueryPushdownClauseList(List *clauseList);
 static bool ExtractFromExpressionWalker(Node *node,
@@ -564,13 +581,10 @@ DeferErrorIfUnsupportedSubqueryPushdown(Query *originalQuery,
 	}
 
 	/* we shouldn't allow reference tables in the outer part of outer joins */
-	if (HasUnsupportedReferenceTableJoin(plannerRestrictionContext))
+	error = DeferredErrorIfUnsupportedRecurringTuplesJoin(plannerRestrictionContext);
+	if (error)
 	{
-		return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
-							 "cannot pushdown the subquery",
-							 "There exist a reference table in the outer part of the "
-							 "outer join",
-							 NULL);
+		return error;
 	}
 
 	/*
@@ -612,7 +626,7 @@ DeferErrorIfUnsupportedSubqueryPushdown(Query *originalQuery,
  * sublinks into joins.
  *
  * In some cases, sublinks are pulled up and converted into outer joins. Those cases
- * are already handled with HasUnsupportedReferenceTableJoin().
+ * are already handled with DeferredErrorIfUnsupportedRecurringTuplesJoin().
  *
  * If the sublinks are not pulled up, we should still error out in if any reference table
  * appears in the FROM clause of a subquery.
@@ -622,19 +636,39 @@ DeferErrorIfUnsupportedSubqueryPushdown(Query *originalQuery,
 static DeferredErrorMessage *
 DeferErrorIfUnsupportedSublinkAndReferenceTable(Query *queryTree)
 {
+	RecurringTuplesType recurType = RECURRING_TUPLES_INVALID;
+
 	if (!queryTree->hasSubLinks)
 	{
 		return NULL;
 	}
 
-	if (HasReferenceTable((Node *) queryTree->rtable))
+	if (HasRecurringTuples((Node *) queryTree->rtable, &recurType))
 	{
-		return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
-							 "cannot pushdown the subquery",
-							 "Reference tables are not allowed in FROM "
-							 "clause when the query has subqueries in "
-							 "WHERE clause",
-							 NULL);
+		if (recurType == RECURRING_TUPLES_REFERENCE_TABLE)
+		{
+			return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
+								 "cannot pushdown the subquery",
+								 "Reference tables are not allowed in FROM "
+								 "clause when the query has subqueries in "
+								 "WHERE clause", NULL);
+		}
+		else if (recurType == RECURRING_TUPLES_FUNCTION)
+		{
+			return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
+								 "cannot pushdown the subquery",
+								 "Functions are not allowed in FROM "
+								 "clause when the query has subqueries in "
+								 "WHERE clause", NULL);
+		}
+		else
+		{
+			return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
+								 "cannot pushdown the subquery",
+								 "Subqueries without FROM are not allowed in FROM "
+								 "clause when the outer query has subqueries in "
+								 "WHERE clause", NULL);
+		}
 	}
 
 	return NULL;
@@ -800,10 +834,12 @@ DeferErrorIfCannotPushdownSubquery(Query *subqueryTree, bool outerMostQueryHasLi
 		return deferredError;
 	}
 
-	if (subqueryTree->rtable == NIL)
+	if (subqueryTree->rtable == NIL &&
+		contain_mutable_functions((Node *) subqueryTree->targetList))
 	{
 		preconditionsSatisfied = false;
-		errorDetail = "Subqueries without relations are unsupported";
+		errorDetail = "Subqueries without a FROM clause can only contain immutable "
+					  "functions";
 	}
 
 	if (subqueryTree->limitOffset)
@@ -948,6 +984,7 @@ DeferErrorIfUnsupportedUnionQuery(Query *subqueryTree,
 {
 	List *setOperationStatementList = NIL;
 	ListCell *setOperationStatmentCell = NULL;
+	RecurringTuplesType recurType = RECURRING_TUPLES_INVALID;
 
 	ExtractSetOperationStatmentWalker((Node *) subqueryTree->setOperations,
 									  &setOperationStatementList);
@@ -973,12 +1010,9 @@ DeferErrorIfUnsupportedUnionQuery(Query *subqueryTree,
 			leftArgRTI = ((RangeTblRef *) leftArg)->rtindex;
 			leftArgSubquery = (Node *) rt_fetch(leftArgRTI,
 												subqueryTree->rtable)->subquery;
-			if (HasReferenceTable(leftArgSubquery))
+			if (HasRecurringTuples(leftArgSubquery, &recurType))
 			{
-				return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
-									 "cannot push down this subquery ",
-									 "Reference tables are not supported with union"
-									 " operator", NULL);
+				break;
 			}
 		}
 
@@ -988,16 +1022,36 @@ DeferErrorIfUnsupportedUnionQuery(Query *subqueryTree,
 			rightArgRTI = ((RangeTblRef *) rightArg)->rtindex;
 			rightArgSubquery = (Node *) rt_fetch(rightArgRTI,
 												 subqueryTree->rtable)->subquery;
-			if (HasReferenceTable(rightArgSubquery))
+			if (HasRecurringTuples(rightArgSubquery, &recurType))
 			{
-				return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
-									 "cannot push down this subquery",
-									 "Reference tables are not supported with union"
-									 " operator", NULL);
+				break;
 			}
 		}
 	}
 
+	if (recurType == RECURRING_TUPLES_REFERENCE_TABLE)
+	{
+		return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
+							 "cannot push down this subquery",
+							 "Reference tables are not supported with union operator",
+							 NULL);
+	}
+	else if (recurType == RECURRING_TUPLES_FUNCTION)
+	{
+		return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
+							 "cannot push down this subquery",
+							 "Table functions are not supported with union operator",
+							 NULL);
+	}
+	else if (recurType == RECURRING_TUPLES_EMPTY_JOIN_TREE)
+	{
+		return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
+							 "cannot push down this subquery",
+							 "Subqueries without a FROM clause are not supported with "
+							 "union operator", NULL);
+	}
+
+
 	return NULL;
 }
 
@@ -1043,16 +1097,15 @@ DeferErrorIfUnsupportedTableCombination(Query *queryTree)
 	List *rangeTableList = queryTree->rtable;
 	List *joinTreeTableIndexList = NIL;
 	ListCell *joinTreeTableIndexCell = NULL;
-	bool unsupporteTableCombination = false;
+	bool unsupportedTableCombination = false;
 	char *errorDetail = NULL;
-	uint32 relationRangeTableCount = 0;
-	uint32 subqueryRangeTableCount = 0;
 
 	/*
 	 * Extract all range table indexes from the join tree. Note that sub-queries
 	 * that get pulled up by PostgreSQL don't appear in this join tree.
 	 */
 	ExtractRangeTableIndexWalker((Node *) queryTree->jointree, &joinTreeTableIndexList);
+
 	foreach(joinTreeTableIndexCell, joinTreeTableIndexList)
 	{
 		/*
@@ -1066,28 +1119,50 @@ DeferErrorIfUnsupportedTableCombination(Query *queryTree)
 			(RangeTblEntry *) list_nth(rangeTableList, rangeTableListIndex);
 
 		/*
-		 * Check if the range table in the join tree is a simple relation or a
-		 * subquery.
+		 * Check if the range table in the join tree is a simple relation, a
+		 * subquery, or immutable function.
 		 */
-		if (rangeTableEntry->rtekind == RTE_RELATION)
+		if (rangeTableEntry->rtekind == RTE_RELATION ||
+			rangeTableEntry->rtekind == RTE_SUBQUERY)
 		{
-			relationRangeTableCount++;
+			/* accepted */
 		}
-		else if (rangeTableEntry->rtekind == RTE_SUBQUERY)
+		else if (rangeTableEntry->rtekind == RTE_FUNCTION)
 		{
-			subqueryRangeTableCount++;
+			if (contain_mutable_functions((Node *) rangeTableEntry->functions))
+			{
+				unsupportedTableCombination = true;
+				errorDetail = "Only immutable functions can be used as a table "
+							  "expressions in a multi-shard query";
+			}
+			else
+			{
+				/* immutable function RTEs are treated as reference tables */
+			}
+		}
+		else if (rangeTableEntry->rtekind == RTE_CTE)
+		{
+			unsupportedTableCombination = true;
+			errorDetail = "CTEs in multi-shard queries are currently unsupported";
+			break;
+		}
+		else if (rangeTableEntry->rtekind == RTE_VALUES)
+		{
+			unsupportedTableCombination = true;
+			errorDetail = "VALUES in multi-shard queries is currently unsupported";
+			break;
 		}
 		else
 		{
-			unsupporteTableCombination = true;
-			errorDetail = "Table expressions other than simple relations and "
-						  "subqueries are currently unsupported";
+			unsupportedTableCombination = true;
+			errorDetail = "Table expressions other than relations, subqueries, "
+						  "and immutable functions are currently unsupported";
 			break;
 		}
 	}
 
 	/* finally check and error out if not satisfied */
-	if (unsupporteTableCombination)
+	if (unsupportedTableCombination)
 	{
 		return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
 							 "cannot push down this subquery",
@@ -1685,7 +1760,7 @@ MultiPlanTree(Query *queryTree)
 
 
 /*
- * HasUnsupportedReferenceTableJoin returns true if there exists a outer join
+ * DeferredErrorIfUnsupportedRecurringTuplesJoin returns true if there exists a outer join
  * between reference table and distributed tables which does not follow
  * the rules :
  * - Reference tables can not be located in the outer part of the semi join or the
@@ -1700,12 +1775,14 @@ MultiPlanTree(Query *queryTree)
  * definitely have duplicate rows. Beside, reference tables can not be used
  * with full outer joins because of the same reason.
  */
-static bool
-HasUnsupportedReferenceTableJoin(PlannerRestrictionContext *plannerRestrictionContext)
+static DeferredErrorMessage *
+DeferredErrorIfUnsupportedRecurringTuplesJoin(
+	PlannerRestrictionContext *plannerRestrictionContext)
 {
 	List *joinRestrictionList =
 		plannerRestrictionContext->joinRestrictionContext->joinRestrictionList;
 	ListCell *joinRestrictionCell = NULL;
+	RecurringTuplesType recurType = RECURRING_TUPLES_INVALID;
 
 	foreach(joinRestrictionCell, joinRestrictionList)
 	{
@@ -1718,36 +1795,62 @@ HasUnsupportedReferenceTableJoin(PlannerRestrictionContext *plannerRestrictionCo
 
 		if (joinType == JOIN_SEMI || joinType == JOIN_ANTI || joinType == JOIN_LEFT)
 		{
-			if (ShouldRecurseForReferenceTableJoinChecks(outerrel) &&
-				RelationInfoContainsReferenceTable(plannerInfo, outerrel))
+			if (ShouldRecurseForRecurringTuplesJoinChecks(outerrel) &&
+				RelationInfoContainsRecurringTuples(plannerInfo, outerrel, &recurType))
 			{
-				return true;
+				break;
 			}
 		}
 		else if (joinType == JOIN_FULL)
 		{
-			if ((ShouldRecurseForReferenceTableJoinChecks(innerrel) &&
-				 RelationInfoContainsReferenceTable(plannerInfo, innerrel)) ||
-				(ShouldRecurseForReferenceTableJoinChecks(outerrel) &&
-				 RelationInfoContainsReferenceTable(plannerInfo, outerrel)))
+			if ((ShouldRecurseForRecurringTuplesJoinChecks(innerrel) &&
+				 RelationInfoContainsRecurringTuples(plannerInfo, innerrel,
+													 &recurType)) ||
+				(ShouldRecurseForRecurringTuplesJoinChecks(outerrel) &&
+				 RelationInfoContainsRecurringTuples(plannerInfo, outerrel, &recurType)))
 			{
-				return true;
+				break;
 			}
 		}
 	}
 
-	return false;
+	if (recurType == RECURRING_TUPLES_REFERENCE_TABLE)
+	{
+		return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
+							 "cannot pushdown the subquery",
+							 "There exist a reference table in the outer "
+							 "part of the outer join", NULL);
+	}
+	else if (recurType == RECURRING_TUPLES_FUNCTION)
+	{
+		return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
+							 "cannot pushdown the subquery",
+							 "There exist a table function in the outer "
+							 "part of the outer join", NULL);
+	}
+	else if (recurType == RECURRING_TUPLES_EMPTY_JOIN_TREE)
+	{
+		return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
+							 "cannot pushdown the subquery",
+							 "There exist a subquery without FROM in the outer "
+							 "part of the outer join", NULL);
+	}
+
+	return NULL;
 }
 
 
 /*
- * ShouldRecurseForReferenceTableJoinChecks is a helper function for deciding
- * on whether the input relOptInfo should be checked for unsupported reference
- * tables.
+ * ShouldRecurseForRecurringTuplesJoinChecks is a helper function for deciding
+ * on whether the input relOptInfo should be checked for table expressions that
+ * generate the same tuples in every query on a shard. We use this to avoid
+ * redundant checks and false positives in complex join trees.
  */
 static bool
-ShouldRecurseForReferenceTableJoinChecks(RelOptInfo *relOptInfo)
+ShouldRecurseForRecurringTuplesJoinChecks(RelOptInfo *relOptInfo)
 {
+	bool shouldRecurse = true;
+
 	/*
 	 * We shouldn't recursively go down for joins since we're already
 	 * going to process each join seperately. Otherwise we'd restrict
@@ -1784,23 +1887,39 @@ ShouldRecurseForReferenceTableJoinChecks(RelOptInfo *relOptInfo)
 
 		if (list_length(subroot->join_rel_list) > 0)
 		{
-			return false;
+			RelOptInfo *subqueryJoin = linitial(subroot->join_rel_list);
+
+			/*
+			 * Subqueries without relations (e.g. SELECT 1) are a little funny.
+			 * They are treated as having a join, but the join is between 0
+			 * relations and won't be in the join restriction list and therefore
+			 * won't be revisited in DeferredErrorIfUnsupportedRecurringTuplesJoin.
+			 *
+			 * We therefore only skip joins with >0 relations.
+			 */
+			if (bms_num_members(subqueryJoin->relids) > 0)
+			{
+				shouldRecurse = false;
+			}
 		}
 	}
 
-	return true;
+	return shouldRecurse;
 }
 
 
 /*
- * RelationInfoContainsReferenceTable checks whether the relationInfo
- * contains any reference tables. If found, the function returns true.
+ * RelationInfoContainsRecurringTuples checks whether the relationInfo
+ * contains any recurring table expression, namely a reference table,
+ * or immutable function. If found, RelationInfoContainsRecurringTuples
+ * returns true.
  *
  * Note that since relation ids of relationInfo indexes to the range
  * table entry list of planner info, planner info is also passed.
  */
 static bool
-RelationInfoContainsReferenceTable(PlannerInfo *plannerInfo, RelOptInfo *relationInfo)
+RelationInfoContainsRecurringTuples(PlannerInfo *plannerInfo, RelOptInfo *relationInfo,
+									RecurringTuplesType *recurType)
 {
 	Relids relids = bms_copy(relationInfo->relids);
 	int relationId = -1;
@@ -1810,7 +1929,7 @@ RelationInfoContainsReferenceTable(PlannerInfo *plannerInfo, RelOptInfo *relatio
 		RangeTblEntry *rangeTableEntry = plannerInfo->simple_rte_array[relationId];
 
 		/* relationInfo has this range table entry */
-		if (HasReferenceTable((Node *) rangeTableEntry))
+		if (HasRecurringTuples((Node *) rangeTableEntry, recurType))
 		{
 			return true;
 		}
@@ -1821,29 +1940,71 @@ RelationInfoContainsReferenceTable(PlannerInfo *plannerInfo, RelOptInfo *relatio
 
 
 /*
- * HasReferenceTable checks whether there exist a reference table in the
- * given node.
+ * HasRecurringTuples returns whether any part of the expression will generate
+ * the same set of tuples in every query on shards when executing a distributed
+ * query.
  */
 bool
-HasReferenceTable(Node *node)
+HasRecurringTuples(Node *node, RecurringTuplesType *recurType)
 {
-	List *relationList = NIL;
-	ListCell *relationCell = NULL;
-	ExtractRangeTableRelationWalkerWithRTEExpand(node, &relationList);
-
-	foreach(relationCell, relationList)
+	if (node == NULL)
 	{
-		RangeTblEntry *rangeTableEntry = (RangeTblEntry *) lfirst(relationCell);
-		Oid relationId = rangeTableEntry->relid;
-
-		if (IsDistributedTable(relationId) && PartitionMethod(relationId) ==
-			DISTRIBUTE_BY_NONE)
-		{
-			return true;
-		}
+		return false;
 	}
 
-	return false;
+	if (IsA(node, RangeTblEntry))
+	{
+		RangeTblEntry *rangeTableEntry = (RangeTblEntry *) node;
+
+		if (rangeTableEntry->rtekind == RTE_RELATION)
+		{
+			Oid relationId = rangeTableEntry->relid;
+			if (IsDistributedTable(relationId) &&
+				PartitionMethod(relationId) == DISTRIBUTE_BY_NONE)
+			{
+				*recurType = RECURRING_TUPLES_REFERENCE_TABLE;
+
+				/*
+				 * Tuples from reference tables will recur in every query on shards
+				 * that includes it.
+				 */
+				return true;
+			}
+		}
+		else if (rangeTableEntry->rtekind == RTE_FUNCTION)
+		{
+			*recurType = RECURRING_TUPLES_FUNCTION;
+
+			/*
+			 * Tuples from functions will recur in every query on shards that includes
+			 * it.
+			 */
+			return true;
+		}
+
+		return range_table_walker(list_make1(rangeTableEntry), HasRecurringTuples,
+								  recurType, 0);
+	}
+	else if (IsA(node, Query))
+	{
+		Query *query = (Query *) node;
+
+		if (query->rtable == NIL)
+		{
+			*recurType = RECURRING_TUPLES_EMPTY_JOIN_TREE;
+
+			/*
+			 * Queries with empty join trees will recur in every query on shards
+			 * that includes it.
+			 */
+			return true;
+		}
+
+		return query_tree_walker((Query *) node, HasRecurringTuples,
+								 recurType, QTW_EXAMINE_RTES);
+	}
+
+	return expression_tree_walker(node, HasRecurringTuples, recurType);
 }
 
 
@@ -3212,54 +3373,6 @@ ExtractRangeTableRelationWalker(Node *node, List **rangeTableRelationList)
 }
 
 
-/*
- * ExtractRangeTableRelationWalkerWithRTEExpand obtains the list of relations
- * from the given node. Note that the difference between this function and
- * ExtractRangeTableRelationWalker is that this one recursively
- * walk into range table entries if it can.
- */
-bool
-ExtractRangeTableRelationWalkerWithRTEExpand(Node *node, List **rangeTableRelationList)
-{
-	bool walkIsComplete = false;
-
-	if (node == NULL)
-	{
-		return walkIsComplete;
-	}
-	else if (IsA(node, RangeTblEntry))
-	{
-		RangeTblEntry *rangeTableEntry = (RangeTblEntry *) node;
-		List *rangeTableList = list_make1(rangeTableEntry);
-
-		if (rangeTableEntry->rtekind == RTE_RELATION)
-		{
-			(*rangeTableRelationList) = lappend(*rangeTableRelationList, rangeTableEntry);
-		}
-		else
-		{
-			walkIsComplete = range_table_walker(rangeTableList,
-												ExtractRangeTableRelationWalkerWithRTEExpand,
-												rangeTableRelationList, 0);
-		}
-	}
-	else if (IsA(node, Query))
-	{
-		walkIsComplete = query_tree_walker((Query *) node,
-										   ExtractRangeTableRelationWalkerWithRTEExpand,
-										   rangeTableRelationList, QTW_EXAMINE_RTES);
-	}
-	else
-	{
-		walkIsComplete = expression_tree_walker(node,
-												ExtractRangeTableRelationWalkerWithRTEExpand,
-												rangeTableRelationList);
-	}
-
-	return walkIsComplete;
-}
-
-
 /*
  * ExtractRangeTableEntryWalker walks over a query tree, and finds all range
  * table entries. For recursing into the query tree, this function uses the
diff --git a/src/backend/distributed/planner/relation_restriction_equivalence.c b/src/backend/distributed/planner/relation_restriction_equivalence.c
index 786df8736..4fc40b535 100644
--- a/src/backend/distributed/planner/relation_restriction_equivalence.c
+++ b/src/backend/distributed/planner/relation_restriction_equivalence.c
@@ -1314,6 +1314,12 @@ ContainsUnionSubquery(Query *queryTree)
 		return false;
 	}
 
+	/* subquery without FROM */
+	if (joiningRangeTableCount == 0)
+	{
+		return false;
+	}
+
 	subqueryRteIndex = linitial_int(joinTreeTableIndexList);
 	rangeTableEntry = rt_fetch(subqueryRteIndex, rangeTableList);
 	if (rangeTableEntry->rtekind != RTE_SUBQUERY)
diff --git a/src/include/distributed/multi_logical_planner.h b/src/include/distributed/multi_logical_planner.h
index 2465b93ad..693902ab4 100644
--- a/src/include/distributed/multi_logical_planner.h
+++ b/src/include/distributed/multi_logical_planner.h
@@ -210,8 +210,6 @@ extern List * TableEntryList(List *rangeTableList);
 extern List * UsedTableEntryList(Query *query);
 extern bool ExtractRangeTableRelationWalker(Node *node, List **rangeTableList);
 extern bool ExtractRangeTableEntryWalker(Node *node, List **rangeTableList);
-extern bool ExtractRangeTableRelationWalkerWithRTEExpand(Node *node,
-														 List **rangeTableList);
 extern List * pull_var_clause_default(Node *node);
 extern bool OperatorImplementsEquality(Oid opno);
 
diff --git a/src/include/distributed/multi_physical_planner.h b/src/include/distributed/multi_physical_planner.h
index 1e16286c4..7b491ca2d 100644
--- a/src/include/distributed/multi_physical_planner.h
+++ b/src/include/distributed/multi_physical_planner.h
@@ -287,7 +287,6 @@ extern Const * MakeInt4Constant(Datum constantValue);
 extern int CompareShardPlacements(const void *leftElement, const void *rightElement);
 extern bool ShardIntervalsOverlap(ShardInterval *firstInterval,
 								  ShardInterval *secondInterval);
-extern bool HasReferenceTable(Node *node);
 
 /* function declarations for Task and Task list operations */
 extern bool TasksEqual(const Task *a, const Task *b);
diff --git a/src/test/regress/expected/multi_insert_select_non_pushable_queries.out b/src/test/regress/expected/multi_insert_select_non_pushable_queries.out
index a1cfaea68..80e6963a2 100644
--- a/src/test/regress/expected/multi_insert_select_non_pushable_queries.out
+++ b/src/test/regress/expected/multi_insert_select_non_pushable_queries.out
@@ -694,7 +694,7 @@ FROM (
         users_table.value_1 > 10 AND users_table.value_1 < 12
         ) u LEFT JOIN LATERAL (
           SELECT event_type, time
-          FROM events_table, (SELECT 1 as x) as f
+          FROM events_table, (SELECT random()::int as x) as f
           WHERE user_id = u.user_id AND
           events_table.event_type > 10 AND events_table.event_type < 12
         ) t ON true
@@ -702,4 +702,4 @@ FROM (
 ) AS shard_union
 ORDER BY user_lastseen DESC;
 ERROR:  cannot push down this subquery
-DETAIL:  Subqueries without relations are unsupported
+DETAIL:  Subqueries without a FROM clause can only contain immutable functions
diff --git a/src/test/regress/expected/multi_subquery.out b/src/test/regress/expected/multi_subquery.out
index 01a87d0d8..981ce47af 100644
--- a/src/test/regress/expected/multi_subquery.out
+++ b/src/test/regress/expected/multi_subquery.out
@@ -71,6 +71,12 @@ FROM
 		l_orderkey) AS unit_prices;
 ERROR:  cannot pushdown the subquery since all relations are not joined using distribution keys
 DETAIL:  Each relation should be joined with at least one another relation using distribution keys and equality operator.
+-- Subqueries without relation with a volatile functions (non-constant)
+SELECT count(*) FROM (
+   SELECT l_orderkey FROM lineitem_subquery JOIN (SELECT random()::int r) sub ON (l_orderkey = r)
+) b;
+ERROR:  cannot push down this subquery
+DETAIL:  Subqueries without a FROM clause can only contain immutable functions
 -- Check that we error out if there is non relation subqueries
 SELECT count(*) FROM
 (
@@ -78,7 +84,7 @@ SELECT count(*) FROM
    (SELECT 1::bigint)
 ) b;
 ERROR:  cannot push down this subquery
-DETAIL:  Subqueries without relations are unsupported
+DETAIL:  Subqueries without a FROM clause are not supported with union operator
 -- Check that we error out if queries in union do not include partition columns.
 SELECT count(*) FROM
 (
diff --git a/src/test/regress/expected/multi_subquery_behavioral_analytics.out b/src/test/regress/expected/multi_subquery_behavioral_analytics.out
index 0ff786728..f47f4b28b 100644
--- a/src/test/regress/expected/multi_subquery_behavioral_analytics.out
+++ b/src/test/regress/expected/multi_subquery_behavioral_analytics.out
@@ -1803,7 +1803,7 @@ ORDER BY 1,2;
 (2 rows)
 
 DROP FUNCTION array_index(ANYARRAY, ANYELEMENT);
--- a not supported query due to constant range table entry
+-- a query with a constant subquery
 SELECT count(*) as subquery_count
 FROM (
   SELECT 
@@ -1822,9 +1822,12 @@ FROM (
   ON a.user_id = b.user_id 
 WHERE b.user_id IS NULL
 GROUP BY a.user_id;
-ERROR:  cannot push down this subquery
-DETAIL:  Subqueries without relations are unsupported
--- same with INNER JOIN
+ subquery_count 
+----------------
+              1
+(1 row)
+
+-- volatile function in the subquery
 SELECT count(*) as subquery_count
 FROM (
   SELECT 
@@ -1838,13 +1841,13 @@ FROM (
   ) as a
   INNER JOIN (
   SELECT
-    1 as user_id
+    random()::int as user_id
   ) AS b 
   ON a.user_id = b.user_id 
 WHERE b.user_id IS NULL
 GROUP BY a.user_id;
 ERROR:  cannot push down this subquery
-DETAIL:  Subqueries without relations are unsupported
+DETAIL:  Subqueries without a FROM clause can only contain immutable functions
 -- this is slightly different, we use RTE_VALUEs here
 SELECT Count(*) AS subquery_count 
 FROM (SELECT 
@@ -1866,7 +1869,7 @@ FROM (SELECT
 WHERE b.user_id IS NULL 
 GROUP BY a.user_id;  
 ERROR:  cannot push down this subquery
-DETAIL:  Table expressions other than simple relations and subqueries are currently unsupported
+DETAIL:  VALUES in multi-shard queries is currently unsupported
 -- same query without LIMIT/OFFSET returns 30 rows
 SET client_min_messages TO DEBUG1;
 -- now, lets use a simple expression on the LIMIT and explicit coercion on the OFFSET
diff --git a/src/test/regress/expected/multi_subquery_complex_queries.out b/src/test/regress/expected/multi_subquery_complex_queries.out
index 6f11c2db1..b4591c669 100644
--- a/src/test/regress/expected/multi_subquery_complex_queries.out
+++ b/src/test/regress/expected/multi_subquery_complex_queries.out
@@ -2440,7 +2440,7 @@ GROUP BY
 ORDER BY 
   types;
 ERROR:  cannot push down this subquery
-DETAIL:  Subqueries without relations are unsupported
+DETAIL:  Subqueries without a FROM clause are not supported with union operator
 -- similar to the above, but constant rte is on the right side of the query
 SELECT ("final_query"."event_types") as types, count(*) AS sumOfEventType
 FROM
@@ -2495,5 +2495,5 @@ GROUP BY
 ORDER BY 
   types;
 ERROR:  cannot push down this subquery
-DETAIL:  Subqueries without relations are unsupported
+DETAIL:  Subqueries without a FROM clause are not supported with union operator
 SET citus.enable_router_execution TO TRUE;
diff --git a/src/test/regress/expected/multi_subquery_complex_reference_clause.out b/src/test/regress/expected/multi_subquery_complex_reference_clause.out
index 3f0ad2069..7dd1b14e6 100644
--- a/src/test/regress/expected/multi_subquery_complex_reference_clause.out
+++ b/src/test/regress/expected/multi_subquery_complex_reference_clause.out
@@ -228,6 +228,91 @@ FROM
       97 | 71002659
 (10 rows)
 
+-- table function can be the inner relationship in a join
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table JOIN generate_series(1,10) AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+ count 
+-------
+    10
+(1 row)
+
+-- table function cannot be used without subquery pushdown
+SELECT count(*) FROM user_buy_test_table JOIN generate_series(1,10) AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id = users_ref_test_table.id;
+ERROR:  could not run distributed query with complex table expressions
+HINT:  Consider using an equality filter on the distributed table's partition column.
+-- table function can be the inner relationship in an outer join
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table LEFT JOIN generate_series(1,10) AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+ count 
+-------
+    10
+(1 row)
+
+-- table function cannot be the outer relationship in an outer join
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table RIGHT JOIN generate_series(1,10) AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+ERROR:  cannot pushdown the subquery
+DETAIL:  There exist a table function in the outer part of the outer join
+-- volatile functions cannot be used as table expressions
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table JOIN random() AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+ERROR:  cannot push down this subquery
+DETAIL:  Only immutable functions can be used as a table expressions in a multi-shard query
+-- cannot sneak in a volatile function as a parameter
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table JOIN generate_series(random()::int,10) AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+ERROR:  cannot push down this subquery
+DETAIL:  Only immutable functions can be used as a table expressions in a multi-shard query
+-- cannot perform a union with table function
+SELECT count(*) FROM
+  (SELECT user_id FROM user_buy_test_table
+   UNION ALL
+   SELECT id FROM generate_series(1,10) AS users_ref_test_table(id)) subquery_1;
+ERROR:  cannot push down this subquery
+DETAIL:  Table functions are not supported with union operator
+-- subquery without FROM can be the inner relationship in a join
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table JOIN (SELECT 4 AS id) users_ref_test_table
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+ count 
+-------
+     1
+(1 row)
+
+-- subquery without FROM triggers subquery pushdown
+SELECT count(*) FROM user_buy_test_table JOIN (SELECT 5 AS id) users_ref_test_table
+ON user_buy_test_table.item_id = users_ref_test_table.id;
+ count 
+-------
+     1
+(1 row)
+
+-- subquery without FROM can be the inner relationship in an outer join
+SELECT count(*) FROM user_buy_test_table LEFT JOIN (SELECT 5 AS id) users_ref_test_table
+ON user_buy_test_table.item_id = users_ref_test_table.id;
+ count 
+-------
+     4
+(1 row)
+
+-- subquery without FROM cannot be the outer relationship in an outer join
+SELECT count(*) FROM user_buy_test_table RIGHT JOIN (SELECT 5 AS id) users_ref_test_table
+ON user_buy_test_table.item_id = users_ref_test_table.id;
+ERROR:  cannot pushdown the subquery
+DETAIL:  There exist a subquery without FROM in the outer part of the outer join
+-- cannot perform a union with subquery without FROM
+SELECT count(*) FROM
+  (SELECT user_id FROM user_buy_test_table
+   UNION ALL
+   SELECT id FROM (SELECT 5 AS id) users_ref_test_table) subquery_1;
+ERROR:  cannot push down this subquery
+DETAIL:  Subqueries without a FROM clause are not supported with union operator
 -- should be able to pushdown since reference table is in the
 -- inner part of the left join
 SELECT
diff --git a/src/test/regress/expected/multi_subquery_in_where_clause.out b/src/test/regress/expected/multi_subquery_in_where_clause.out
index eaa0254b0..a42ce49f4 100644
--- a/src/test/regress/expected/multi_subquery_in_where_clause.out
+++ b/src/test/regress/expected/multi_subquery_in_where_clause.out
@@ -499,16 +499,34 @@ FROM (
 ORDER BY 2 DESC, 1;
 ERROR:  cannot pushdown the subquery since all relations are not joined using distribution keys
 DETAIL:  Each relation should be joined with at least one another relation using distribution keys and equality operator.
--- subquery in where clause doesn't have a relation
-SELECT 
+-- subquery in where clause doesn't have a relation, but is constant
+SELECT
   user_id
-FROM 
+FROM
   users_table
-WHERE 
-  value_2 >  
-          (SELECT 1);
+WHERE
+  value_2 >
+          (SELECT 1)
+ORDER BY 1 ASC
+LIMIT 2;
+ user_id 
+---------
+       0
+       0
+(2 rows)
+
+-- subquery in where clause has a volatile function and no relation
+SELECT
+  user_id
+FROM
+  users_table
+WHERE
+  value_2 >
+          (SELECT random())
+ORDER BY 1 ASC
+LIMIT 2;
 ERROR:  cannot push down this subquery
-DETAIL:  Subqueries without relations are unsupported
+DETAIL:  Subqueries without a FROM clause can only contain immutable functions
 -- OFFSET is not supported in the subquey
 SELECT 
   user_id
diff --git a/src/test/regress/expected/multi_subquery_in_where_reference_clause.out b/src/test/regress/expected/multi_subquery_in_where_reference_clause.out
index ec1399f65..cf3f46876 100644
--- a/src/test/regress/expected/multi_subquery_in_where_reference_clause.out
+++ b/src/test/regress/expected/multi_subquery_in_where_reference_clause.out
@@ -64,6 +64,40 @@ WHERE
 LIMIT 3;
 ERROR:  cannot pushdown the subquery
 DETAIL:  Reference tables are not allowed in FROM clause when the query has subqueries in WHERE clause
+-- immutable functions are also treated as reference tables
+SELECT
+  user_id
+FROM
+  (SELECT user_id FROM generate_series(1,10) AS series(user_id)) users_reference_table
+WHERE
+  NOT EXISTS
+      (SELECT
+          value_2
+       FROM
+          events_table
+       WHERE
+          users_reference_table.user_id = events_table.user_id
+      )
+LIMIT 3;
+ERROR:  cannot pushdown the subquery
+DETAIL:  Functions are not allowed in FROM clause when the query has subqueries in WHERE clause
+-- subqueries without FROM are also treated as reference tables
+SELECT
+  user_id
+FROM
+  (SELECT  5 AS user_id) users_reference_table
+WHERE
+  NOT EXISTS
+      (SELECT
+          value_2
+       FROM
+          events_table
+       WHERE
+          users_reference_table.user_id = events_table.user_id
+      )
+LIMIT 3;
+ERROR:  cannot pushdown the subquery
+DETAIL:  Subqueries without FROM are not allowed in FROM clause when the outer query has subqueries in WHERE clause
 -- subqueries in WHERE with IN operator without equality
 SELECT 
   users_table.user_id, count(*)
@@ -88,6 +122,54 @@ LIMIT 3;
       46 |   115
 (3 rows)
 
+-- immutable functions are also treated as reference tables
+SELECT
+  users_table.user_id, count(*)
+FROM
+  users_table
+WHERE
+  value_2 IN
+          (SELECT
+              value_2
+           FROM
+              generate_series(1,10) AS events_reference_table(user_id)
+           WHERE
+              users_table.user_id > events_reference_table.user_id
+          )
+GROUP BY users_table.user_id
+ORDER BY 2 DESC, 1 DESC
+LIMIT 3;
+ user_id | count 
+---------+-------
+      12 |   121
+      87 |   117
+      59 |   115
+(3 rows)
+
+-- immutable functions are also treated as reference tables
+SELECT
+  users_table.user_id, count(*)
+FROM
+  users_table
+WHERE
+  value_2 IN
+          (SELECT
+              value_2
+           FROM
+              (SELECT 5 AS user_id) AS events_reference_table
+           WHERE
+              users_table.user_id > events_reference_table.user_id
+          )
+GROUP BY users_table.user_id
+ORDER BY 2 DESC, 1 DESC
+LIMIT 3;
+ user_id | count 
+---------+-------
+      12 |   121
+      87 |   117
+      59 |   115
+(3 rows)
+
 -- should error out since reference table exist on the left side 
 -- of the left lateral join
 SELECT user_id, value_2 FROM users_table WHERE
diff --git a/src/test/regress/expected/multi_subquery_union.out b/src/test/regress/expected/multi_subquery_union.out
index eddb00f1f..69acc4edc 100644
--- a/src/test/regress/expected/multi_subquery_union.out
+++ b/src/test/regress/expected/multi_subquery_union.out
@@ -914,7 +914,7 @@ FROM
   (SELECT 1)
 ) b;
 ERROR:  cannot push down this subquery
-DETAIL:  Subqueries without relations are unsupported
+DETAIL:  Subqueries without a FROM clause are not supported with union operator
 -- we don't support subqueries without relations
 SELECT 
   *
@@ -925,7 +925,7 @@ FROM
   (SELECT (random() * 100)::int)
 ) b;
 ERROR:  cannot push down this subquery
-DETAIL:  Subqueries without relations are unsupported
+DETAIL:  Subqueries without a FROM clause are not supported with union operator
 -- we don't support subqueries without relations
 SELECT 
   user_id, value_3
@@ -946,7 +946,7 @@ FROM
 ORDER BY 1 DESC, 2 DESC
 LIMIT 5;
 ERROR:  cannot push down this subquery
-DETAIL:  Subqueries without relations are unsupported
+DETAIL:  Subqueries without a FROM clause are not supported with union operator
 SELECT ("final_query"."event_types") as types, count(*) AS sumOfEventType
 FROM
   ( SELECT *, random()
@@ -990,7 +990,7 @@ FROM
 GROUP BY types
 ORDER BY types;
 ERROR:  cannot push down this subquery
-DETAIL:  Subqueries without relations are unsupported
+DETAIL:  Subqueries without a FROM clause are not supported with union operator
 SET citus.enable_router_execution TO true;
 DROP TABLE events_reference_table;
 DROP TABLE users_reference_table;
diff --git a/src/test/regress/expected/multi_view.out b/src/test/regress/expected/multi_view.out
index 50c0a5039..dd55fb597 100644
--- a/src/test/regress/expected/multi_view.out
+++ b/src/test/regress/expected/multi_view.out
@@ -715,12 +715,12 @@ CREATE VIEW cte_view_1 AS
 WITH c1 AS (SELECT * FROM users_table WHERE value_1 = 15) SELECT * FROM c1 WHERE value_2 < 500;
 SELECT * FROM cte_view_1;
 ERROR:  cannot push down this subquery
-DETAIL:  Table expressions other than simple relations and subqueries are currently unsupported
+DETAIL:  CTEs in multi-shard queries are currently unsupported
 -- this is single shard query but still not supported since it has view + cte
 -- router planner can't detect it
 SELECT * FROM cte_view_1 WHERE user_id = 8;
 ERROR:  cannot push down this subquery
-DETAIL:  Table expressions other than simple relations and subqueries are currently unsupported
+DETAIL:  CTEs in multi-shard queries are currently unsupported
 -- if CTE itself prunes down to a single shard than the view is supported (router plannable)
 CREATE VIEW cte_view_2 AS
 WITH c1 AS (SELECT * FROM users_table WHERE user_id = 8) SELECT * FROM c1 WHERE value_1 = 15;
diff --git a/src/test/regress/sql/multi_insert_select_non_pushable_queries.sql b/src/test/regress/sql/multi_insert_select_non_pushable_queries.sql
index a4fb4d06b..98159f36b 100644
--- a/src/test/regress/sql/multi_insert_select_non_pushable_queries.sql
+++ b/src/test/regress/sql/multi_insert_select_non_pushable_queries.sql
@@ -681,7 +681,7 @@ FROM (
         users_table.value_1 > 10 AND users_table.value_1 < 12
         ) u LEFT JOIN LATERAL (
           SELECT event_type, time
-          FROM events_table, (SELECT 1 as x) as f
+          FROM events_table, (SELECT random()::int as x) as f
           WHERE user_id = u.user_id AND
           events_table.event_type > 10 AND events_table.event_type < 12
         ) t ON true
diff --git a/src/test/regress/sql/multi_subquery.sql b/src/test/regress/sql/multi_subquery.sql
index 800e5389c..2b2afd586 100644
--- a/src/test/regress/sql/multi_subquery.sql
+++ b/src/test/regress/sql/multi_subquery.sql
@@ -70,6 +70,11 @@ FROM
 	GROUP BY
 		l_orderkey) AS unit_prices;
 
+-- Subqueries without relation with a volatile functions (non-constant)
+SELECT count(*) FROM (
+   SELECT l_orderkey FROM lineitem_subquery JOIN (SELECT random()::int r) sub ON (l_orderkey = r)
+) b;
+
 -- Check that we error out if there is non relation subqueries
 SELECT count(*) FROM
 (
diff --git a/src/test/regress/sql/multi_subquery_behavioral_analytics.sql b/src/test/regress/sql/multi_subquery_behavioral_analytics.sql
index 051a1265f..2d8ed41b1 100644
--- a/src/test/regress/sql/multi_subquery_behavioral_analytics.sql
+++ b/src/test/regress/sql/multi_subquery_behavioral_analytics.sql
@@ -1432,7 +1432,7 @@ SELECT * FROM run_command_on_workers('DROP FUNCTION array_index(ANYARRAY, ANYELE
 ORDER BY 1,2;
 DROP FUNCTION array_index(ANYARRAY, ANYELEMENT);
 
--- a not supported query due to constant range table entry
+-- a query with a constant subquery
 SELECT count(*) as subquery_count
 FROM (
   SELECT 
@@ -1452,7 +1452,7 @@ FROM (
 WHERE b.user_id IS NULL
 GROUP BY a.user_id;
 
--- same with INNER JOIN
+-- volatile function in the subquery
 SELECT count(*) as subquery_count
 FROM (
   SELECT 
@@ -1466,7 +1466,7 @@ FROM (
   ) as a
   INNER JOIN (
   SELECT
-    1 as user_id
+    random()::int as user_id
   ) AS b 
   ON a.user_id = b.user_id 
 WHERE b.user_id IS NULL
diff --git a/src/test/regress/sql/multi_subquery_complex_reference_clause.sql b/src/test/regress/sql/multi_subquery_complex_reference_clause.sql
index 731daa60a..55dbb2e30 100644
--- a/src/test/regress/sql/multi_subquery_complex_reference_clause.sql
+++ b/src/test/regress/sql/multi_subquery_complex_reference_clause.sql
@@ -136,6 +136,64 @@ FROM
   ) as foo
   GROUP BY user_id ORDER BY 2 DESC LIMIT 10;
 
+-- table function can be the inner relationship in a join
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table JOIN generate_series(1,10) AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+
+-- table function cannot be used without subquery pushdown
+SELECT count(*) FROM user_buy_test_table JOIN generate_series(1,10) AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id = users_ref_test_table.id;
+
+-- table function can be the inner relationship in an outer join
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table LEFT JOIN generate_series(1,10) AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+
+-- table function cannot be the outer relationship in an outer join
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table RIGHT JOIN generate_series(1,10) AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+
+-- volatile functions cannot be used as table expressions
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table JOIN random() AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+
+-- cannot sneak in a volatile function as a parameter
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table JOIN generate_series(random()::int,10) AS users_ref_test_table(id)
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+
+-- cannot perform a union with table function
+SELECT count(*) FROM
+  (SELECT user_id FROM user_buy_test_table
+   UNION ALL
+   SELECT id FROM generate_series(1,10) AS users_ref_test_table(id)) subquery_1;
+
+-- subquery without FROM can be the inner relationship in a join
+SELECT count(*) FROM
+  (SELECT random() FROM user_buy_test_table JOIN (SELECT 4 AS id) users_ref_test_table
+  ON user_buy_test_table.item_id > users_ref_test_table.id) subquery_1;
+
+-- subquery without FROM triggers subquery pushdown
+SELECT count(*) FROM user_buy_test_table JOIN (SELECT 5 AS id) users_ref_test_table
+ON user_buy_test_table.item_id = users_ref_test_table.id;
+
+-- subquery without FROM can be the inner relationship in an outer join
+SELECT count(*) FROM user_buy_test_table LEFT JOIN (SELECT 5 AS id) users_ref_test_table
+ON user_buy_test_table.item_id = users_ref_test_table.id;
+
+-- subquery without FROM cannot be the outer relationship in an outer join
+SELECT count(*) FROM user_buy_test_table RIGHT JOIN (SELECT 5 AS id) users_ref_test_table
+ON user_buy_test_table.item_id = users_ref_test_table.id;
+
+-- cannot perform a union with subquery without FROM
+SELECT count(*) FROM
+  (SELECT user_id FROM user_buy_test_table
+   UNION ALL
+   SELECT id FROM (SELECT 5 AS id) users_ref_test_table) subquery_1;
+
 -- should be able to pushdown since reference table is in the
 -- inner part of the left join
 SELECT
diff --git a/src/test/regress/sql/multi_subquery_in_where_clause.sql b/src/test/regress/sql/multi_subquery_in_where_clause.sql
index 6d60a2c1b..472e1bc3c 100644
--- a/src/test/regress/sql/multi_subquery_in_where_clause.sql
+++ b/src/test/regress/sql/multi_subquery_in_where_clause.sql
@@ -433,14 +433,27 @@ FROM (
 ) q
 ORDER BY 2 DESC, 1;
 
--- subquery in where clause doesn't have a relation
-SELECT 
+-- subquery in where clause doesn't have a relation, but is constant
+SELECT
   user_id
-FROM 
+FROM
   users_table
-WHERE 
-  value_2 >  
-          (SELECT 1);
+WHERE
+  value_2 >
+          (SELECT 1)
+ORDER BY 1 ASC
+LIMIT 2;
+
+-- subquery in where clause has a volatile function and no relation
+SELECT
+  user_id
+FROM
+  users_table
+WHERE
+  value_2 >
+          (SELECT random())
+ORDER BY 1 ASC
+LIMIT 2;
 
 -- OFFSET is not supported in the subquey
 SELECT 
diff --git a/src/test/regress/sql/multi_subquery_in_where_reference_clause.sql b/src/test/regress/sql/multi_subquery_in_where_reference_clause.sql
index c996df7af..8bc716cd0 100644
--- a/src/test/regress/sql/multi_subquery_in_where_reference_clause.sql
+++ b/src/test/regress/sql/multi_subquery_in_where_reference_clause.sql
@@ -55,6 +55,38 @@ WHERE
       )
 LIMIT 3;
 
+-- immutable functions are also treated as reference tables
+SELECT
+  user_id
+FROM
+  (SELECT user_id FROM generate_series(1,10) AS series(user_id)) users_reference_table
+WHERE
+  NOT EXISTS
+      (SELECT
+          value_2
+       FROM
+          events_table
+       WHERE
+          users_reference_table.user_id = events_table.user_id
+      )
+LIMIT 3;
+
+-- subqueries without FROM are also treated as reference tables
+SELECT
+  user_id
+FROM
+  (SELECT  5 AS user_id) users_reference_table
+WHERE
+  NOT EXISTS
+      (SELECT
+          value_2
+       FROM
+          events_table
+       WHERE
+          users_reference_table.user_id = events_table.user_id
+      )
+LIMIT 3;
+
 -- subqueries in WHERE with IN operator without equality
 SELECT 
   users_table.user_id, count(*)
@@ -73,6 +105,43 @@ GROUP BY users_table.user_id
 ORDER BY 2 DESC, 1 DESC
 LIMIT 3;
 
+-- immutable functions are also treated as reference tables
+SELECT
+  users_table.user_id, count(*)
+FROM
+  users_table
+WHERE
+  value_2 IN
+          (SELECT
+              value_2
+           FROM
+              generate_series(1,10) AS events_reference_table(user_id)
+           WHERE
+              users_table.user_id > events_reference_table.user_id
+          )
+GROUP BY users_table.user_id
+ORDER BY 2 DESC, 1 DESC
+LIMIT 3;
+
+-- immutable functions are also treated as reference tables
+SELECT
+  users_table.user_id, count(*)
+FROM
+  users_table
+WHERE
+  value_2 IN
+          (SELECT
+              value_2
+           FROM
+              (SELECT 5 AS user_id) AS events_reference_table
+           WHERE
+              users_table.user_id > events_reference_table.user_id
+          )
+GROUP BY users_table.user_id
+ORDER BY 2 DESC, 1 DESC
+LIMIT 3;
+
+
 -- should error out since reference table exist on the left side 
 -- of the left lateral join
 SELECT user_id, value_2 FROM users_table WHERE