diff --git a/src/backend/distributed/planner/insert_select_planner.c b/src/backend/distributed/planner/insert_select_planner.c
index 1cf996b77..1cd4161b6 100644
--- a/src/backend/distributed/planner/insert_select_planner.c
+++ b/src/backend/distributed/planner/insert_select_planner.c
@@ -428,11 +428,10 @@ CreateInsertSelectIntoLocalTablePlan(uint64 planId, Query *insertSelectQuery,
 									 ParamListInfo boundParams, bool hasUnresolvedParams,
 									 PlannerRestrictionContext *plannerRestrictionContext)
 {
-	RangeTblEntry *selectRte = ExtractSelectRangeTableEntry(insertSelectQuery);
-
 	PrepareInsertSelectForCitusPlanner(insertSelectQuery);
 
 	/* get the SELECT query (may have changed after PrepareInsertSelectForCitusPlanner) */
+	RangeTblEntry *selectRte = ExtractSelectRangeTableEntry(insertSelectQuery);
 	Query *selectQuery = selectRte->subquery;
 
 	bool allowRecursivePlanning = true;
@@ -513,6 +512,24 @@ PrepareInsertSelectForCitusPlanner(Query *insertSelectQuery)
 
 	bool isWrapped = false;
 
+#if PG_VERSION_NUM >= PG_VERSION_18
+
+/*
+ * PG18 is stricter about GroupRTE/GroupVar. For INSERT … SELECT with a GROUP BY,
+ * flatten the SELECT’s targetList and havingQual so Vars point to base RTEs and
+ * avoid Unrecognized range table id.
+ */
+	if (selectRte->subquery->hasGroupRTE)
+	{
+		Query *selectQuery = selectRte->subquery;
+		selectQuery->targetList = (List *)
+								  flatten_group_exprs(NULL, selectQuery,
+													  (Node *) selectQuery->targetList);
+		selectQuery->havingQual =
+			flatten_group_exprs(NULL, selectQuery, selectQuery->havingQual);
+	}
+#endif
+
 	if (selectRte->subquery->setOperations != NULL)
 	{
 		/*
@@ -1431,11 +1448,6 @@ static DistributedPlan *
 CreateNonPushableInsertSelectPlan(uint64 planId, Query *parse, ParamListInfo boundParams)
 {
 	Query *insertSelectQuery = copyObject(parse);
-
-	RangeTblEntry *selectRte = ExtractSelectRangeTableEntry(insertSelectQuery);
-	RangeTblEntry *insertRte = ExtractResultRelationRTEOrError(insertSelectQuery);
-	Oid targetRelationId = insertRte->relid;
-
 	DistributedPlan *distributedPlan = CitusMakeNode(DistributedPlan);
 	distributedPlan->modLevel = RowModifyLevelForQuery(insertSelectQuery);
 
@@ -1450,6 +1462,7 @@ CreateNonPushableInsertSelectPlan(uint64 planId, Query *parse, ParamListInfo bou
 	PrepareInsertSelectForCitusPlanner(insertSelectQuery);
 
 	/* get the SELECT query (may have changed after PrepareInsertSelectForCitusPlanner) */
+	RangeTblEntry *selectRte = ExtractSelectRangeTableEntry(insertSelectQuery);
 	Query *selectQuery = selectRte->subquery;
 
 	/*
@@ -1472,6 +1485,9 @@ CreateNonPushableInsertSelectPlan(uint64 planId, Query *parse, ParamListInfo bou
 	PlannedStmt *selectPlan = pg_plan_query(selectQueryCopy, NULL, cursorOptions,
 											boundParams);
 
+	/* decide whether we can repartition the results */
+	RangeTblEntry *insertRte = ExtractResultRelationRTEOrError(insertSelectQuery);
+	Oid targetRelationId = insertRte->relid;
 	bool repartitioned = IsRedistributablePlan(selectPlan->planTree) &&
 						 IsSupportedRedistributionTarget(targetRelationId);
 
diff --git a/src/backend/distributed/utils/colocation_utils.c b/src/backend/distributed/utils/colocation_utils.c
index 98ada2346..d95fe519e 100644
--- a/src/backend/distributed/utils/colocation_utils.c
+++ b/src/backend/distributed/utils/colocation_utils.c
@@ -73,34 +73,8 @@ PG_FUNCTION_INFO_V1(update_distributed_table_colocation);
 Datum
 mark_tables_colocated(PG_FUNCTION_ARGS)
 {
-	CheckCitusVersion(ERROR);
-	EnsureCoordinator();
-
-	Oid sourceRelationId = PG_GETARG_OID(0);
-	ArrayType *relationIdArrayObject = PG_GETARG_ARRAYTYPE_P(1);
-
-	int relationCount = ArrayObjectCount(relationIdArrayObject);
-	if (relationCount < 1)
-	{
-		ereport(ERROR, (errmsg("at least one target table is required for this "
-							   "operation")));
-	}
-
-	EnsureTableOwner(sourceRelationId);
-
-	Datum *relationIdDatumArray = DeconstructArrayObject(relationIdArrayObject);
-
-	for (int relationIndex = 0; relationIndex < relationCount; relationIndex++)
-	{
-		Oid nextRelationOid = DatumGetObjectId(relationIdDatumArray[relationIndex]);
-
-		/* we require that the user either owns all tables or is superuser */
-		EnsureTableOwner(nextRelationOid);
-
-		MarkTablesColocated(sourceRelationId, nextRelationOid);
-	}
-
-	PG_RETURN_VOID();
+	ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					errmsg("this function is deprecated and no longer is used")));
 }
 
 
@@ -1306,7 +1280,7 @@ ColocatedShardIdInRelation(Oid relationId, int shardIndex)
 /*
  * DeleteColocationGroupIfNoTablesBelong function deletes given co-location group if there
  * is no relation in that co-location group. A co-location group may become empty after
- * mark_tables_colocated or upgrade_reference_table UDF calls. In that case we need to
+ * update_distributed_table_colocation UDF calls. In that case we need to
  * remove empty co-location group to prevent orphaned co-location groups.
  */
 void
diff --git a/src/test/regress/expected/multi_reference_table.out b/src/test/regress/expected/multi_reference_table.out
index 68835be40..b86606f7c 100644
--- a/src/test/regress/expected/multi_reference_table.out
+++ b/src/test/regress/expected/multi_reference_table.out
@@ -1200,7 +1200,7 @@ DEBUG:  cannot perform distributed INSERT INTO ... SELECT because the partition
 DETAIL:  The target table's partition column should correspond to a partition column in the subquery.
 DEBUG:  performing repartitioned INSERT ... SELECT
 RESET client_min_messages;
--- some tests for mark_tables_colocated
+-- some tests for update_distributed_table_colocation
 -- should error out
 SELECT update_distributed_table_colocation('colocated_table_test_2', colocate_with => 'reference_table_test');
 ERROR:  relation reference_table_test should be a hash or single shard distributed table
diff --git a/src/test/regress/sql/multi_reference_table.sql b/src/test/regress/sql/multi_reference_table.sql
index d538effe6..924a135b0 100644
--- a/src/test/regress/sql/multi_reference_table.sql
+++ b/src/test/regress/sql/multi_reference_table.sql
@@ -768,7 +768,7 @@ WHERE
 
 RESET client_min_messages;
 
--- some tests for mark_tables_colocated
+-- some tests for update_distributed_table_colocation
 -- should error out
 SELECT update_distributed_table_colocation('colocated_table_test_2', colocate_with => 'reference_table_test');