Fix ORDER BY lost after AQUMV join rewrite

The rewrite cleared sortClause, so grouping_planner() skipped adding
a Sort node — queries with ORDER BY returned unsorted results from
the MV scan.

Fix: preserve sortClause and copy ressortgroupref to rewritten target
entries so the upper planner generates Sort correctly.

Before: Limit -> Gather -> Limit -> Seq Scan on mv
After:  Limit -> Gather -> Limit -> Sort -> Seq Scan on mv

Author: avamingli

src/backend/optimizer/plan/aqumv.c

@@ -1206,6 +1206,7 @@ answer_query_using_materialized_views_for_join(PlannerInfo *root, AqumvContext a
 										  (AttrNumber) attnum,
 										  old_tle->resname,
 										  false);
+				new_tle->ressortgroupref = old_tle->ressortgroupref;
 				new_tlist = lappend(new_tlist, new_tle);
 			}
 
@@ -1243,11 +1244,11 @@ answer_query_using_materialized_views_for_join(PlannerInfo *root, AqumvContext a
 		viewQuery->jointree = makeFromExpr(list_make1(makeNode(RangeTblRef)), NULL);
 		((RangeTblRef *) linitial(viewQuery->jointree->fromlist))->rtindex = 1;
 
-		/* Clear aggregation/grouping/sorting state — all materialized. */
+		/* Clear aggregation/grouping state — already materialized in MV. */
 		viewQuery->hasAggs = false;
 		viewQuery->groupClause = NIL;
 		viewQuery->havingQual = NULL;
-		viewQuery->sortClause = NIL;
+		/* Keep sortClause: upper planner needs it to add Sort node. */
 		viewQuery->distinctClause = NIL;
 		viewQuery->hasDistinctOn = false;
 		viewQuery->hasWindowFuncs = false;

src/test/regress/expected/matview_data.out

@@ -2077,14 +2077,17 @@ explain(costs off)
   from aqj_orders o join aqj_customers c on o.customer_id = c.customer_id
   where o.status = 'shipped'
   order by o.order_id limit 5;
-                   QUERY PLAN                    
--------------------------------------------------
+                      QUERY PLAN                       
+-------------------------------------------------------
  Limit
    ->  Gather Motion 3:1  (slice1; segments: 3)
+         Merge Key: order_id
          ->  Limit
-               ->  Seq Scan on mv_aqj_limit_test
+               ->  Sort
+                     Sort Key: order_id
+                     ->  Seq Scan on mv_aqj_limit_test
  Optimizer: Postgres query optimizer
-(5 rows)
+(8 rows)
 
 -- 27. Match: FETCH FIRST WITH TIES exact match
 create materialized view mv_aqj_with_ties as
@@ -2138,14 +2141,17 @@ explain(costs off)
   from aqj_orders o join aqj_customers c on o.customer_id = c.customer_id
   where o.status = 'pending'
   order by o.order_id fetch first 5 rows with ties;
-                   QUERY PLAN                   
-------------------------------------------------
+                      QUERY PLAN                      
+------------------------------------------------------
  Limit
    ->  Gather Motion 3:1  (slice1; segments: 3)
+         Merge Key: order_id
          ->  Limit
-               ->  Seq Scan on mv_aqj_with_ties
+               ->  Sort
+                     Sort Key: order_id
+                     ->  Seq Scan on mv_aqj_with_ties
  Optimizer: Postgres query optimizer
-(5 rows)
+(8 rows)
 
 select o.order_id, o.amount
   from aqj_orders o join aqj_customers c on o.customer_id = c.customer_id