If it includes {order by}, ROWNUM can not be rewrited to LIMIT

src/gausskernel/optimizer/prep/prepjointree.cpp

@@ -2963,7 +2963,10 @@ void preprocess_rownum(PlannerInfo *root, Query *parse)
     if (quals == NULL) {
         return;
     }
-
+    /* If it includes {order by}, can not be rewrited */
+    if (parse->sortClause != NULL) {
+        return;
+    }
     if (parse->limitCount != NULL) {
         parse->limitCount = eval_const_expressions(root, parse->limitCount);
         if (!IsA(parse->limitCount, Const)) {

src/test/regress/expected/xc_rownum.out

@@ -1634,5 +1634,16 @@ explain select * from student where not(rownum > 3 or id = 1);
 (3 rows)
 
 
+-- ROWNUM with ORDER BY
+explain select * from test where rownum < 5 order by 1;
+                          QUERY PLAN                          
+--------------------------------------------------------------
+ Sort  (cost=40.36..41.30 rows=378 width=42)
+   Sort Key: id
+   ->  Seq Scan on test  (cost=0.00..24.18 rows=378 width=42)
+         Filter: (ROWNUM < 5)
+(4 rows)
+
+
 drop table student;
 drop table test;

src/test/regress/sql/xc_rownum.sql

@@ -444,5 +444,8 @@ explain select * from student where not(rownum < 3 + 2);
 explain select * from student where not(rownum < 3 and id = 1);
 explain select * from student where not(rownum > 3 or id = 1);
 
+-- ROWNUM with ORDER BY
+explain select * from test where rownum < 5 order by 1;
+
 drop table student;
 drop table test;