Postgres 18: Fix regress tests caused by GROUP RTE. (#8206)

The change in `merge_planner.c` fixes _unrecognized range table entry_
diffs in merge regress tests (category 2 diffs in #7992), the change in
`multi_router_planner.c` fixes _column reference ... is ambiguous_ diffs
in `multi_insert_select` and `multi_insert_select_window` (category 3
diffs in #7992). Edit to `common.py` enables standalone regress tests
with pg18 (e..g `citus_tests/run_test.py merge`).

src/backend/distributed/planner/merge_planner.c

@@ -422,10 +422,13 @@ ErrorIfMergeHasUnsupportedTables(Oid targetRelationId, List *rangeTableList)
 			case RTE_VALUES:
 			case RTE_JOIN:
 			case RTE_CTE:
-			{
-				/* Skip them as base table(s) will be checked */
-				continue;
-			}
+#if PG_VERSION_NUM >= PG_VERSION_18
+			case RTE_GROUP:
+#endif
+				{
+					/* Skip them as base table(s) will be checked */
+					continue;
+				}
 
 			/*
 			 * RTE_NAMEDTUPLESTORE is typically used in ephmeral named relations,

src/backend/distributed/planner/multi_router_planner.c

@@ -372,6 +372,25 @@ AddPartitionKeyNotNullFilterToSelect(Query *subqery)
 	/* we should have found target partition column */
 	Assert(targetPartitionColumnVar != NULL);
 
+#if PG_VERSION_NUM >= PG_VERSION_18
+	if (subqery->hasGroupRTE)
+	{
+		/* if the partition column is a grouped column, we need to flatten it
+		 * to ensure query deparsing works correctly. We choose to do this here
+		 * instead of in ruletils.c because we want to keep the flattening logic
+		 * close to the NOT NULL filter injection.
+		 */
+		RangeTblEntry *partitionRTE = rt_fetch(targetPartitionColumnVar->varno,
+											   subqery->rtable);
+		if (partitionRTE->rtekind == RTE_GROUP)
+		{
+			targetPartitionColumnVar = (Var *) flatten_group_exprs(NULL, subqery,
+																   (Node *)
+																   targetPartitionColumnVar);
+		}
+	}
+#endif
+
 	/* create expression for partition_column IS NOT NULL */
 	NullTest *nullTest = makeNode(NullTest);
 	nullTest->nulltesttype = IS_NOT_NULL;