diff --git a/src/common/backend/parser/parse_agg.cpp b/src/common/backend/parser/parse_agg.cpp
index 2a0311f2d..5a29d8b5f 100755
--- a/src/common/backend/parser/parse_agg.cpp
+++ b/src/common/backend/parser/parse_agg.cpp
@@ -56,6 +56,7 @@ static void finalize_grouping_exprs(
 
 static bool finalize_grouping_exprs_walker(Node* node, check_ungrouped_columns_context* context);
 static List* expand_groupingset_node(GroupingSet* gs);
+static void find_rownum_in_groupby_clauses(Rownum *rownumVar, check_ungrouped_columns_context* context);
 
 /*
  * transformAggregateCall -
@@ -742,23 +743,7 @@ static bool check_ungrouped_columns_walker(Node* node, check_ungrouped_columns_c
 
     /* If There is ROWNUM, it must appear in the GROUP BY clause or be used in an aggregate function. */
     if (IsA(node, Rownum)) {
-        Rownum *rownumVar = (Rownum *)node;
-        bool haveRownum = false;
-        if (!context->have_non_var_grouping || context->sublevels_up != 0) {
-            foreach (gl, context->groupClauses) {
-                Node *gnode = (Node *)((TargetEntry *)lfirst(gl))->expr;
-                if (IsA(gnode, Rownum)) {
-                    haveRownum = true;
-                    break;
-                }
-            }
-
-            if (haveRownum == false) {
-                ereport(ERROR, (errcode(ERRCODE_GROUPING_ERROR),
-                    errmsg("ROWNUM must appear in the GROUP BY clause or be used in an aggregate function"),
-                    parser_errposition(context->pstate, rownumVar->location)));
-            }
-        }
+        find_rownum_in_groupby_clauses((Rownum *)node, context);
     }
     /*
      * If we have an ungrouped Var of the original query level, we have a
@@ -1493,3 +1478,25 @@ Oid resolve_aggregate_transtype(Oid aggfuncid, Oid aggtranstype, Oid* inputTypes
     }
     return aggtranstype;
 }
+
+static void find_rownum_in_groupby_clauses(Rownum *rownumVar, check_ungrouped_columns_context *context)
+{
+    bool haveRownum = false;
+    ListCell *gl = NULL;
+
+    if (!context->have_non_var_grouping || context->sublevels_up != 0) {
+        foreach (gl, context->groupClauses) {
+            Node *gnode = (Node *)((TargetEntry *)lfirst(gl))->expr;
+            if (IsA(gnode, Rownum)) {
+                haveRownum = true;
+                break;
+            }
+        }
+
+        if (haveRownum == false) {
+            ereport(ERROR, (errcode(ERRCODE_GROUPING_ERROR),
+                errmsg("ROWNUM must appear in the GROUP BY clause or be used in an aggregate function"),
+                parser_errposition(context->pstate, rownumVar->location)));
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/gausskernel/optimizer/prep/prepnonjointree.cpp b/src/gausskernel/optimizer/prep/prepnonjointree.cpp
index 2a3b99910..3149cdb2b 100755
--- a/src/gausskernel/optimizer/prep/prepnonjointree.cpp
+++ b/src/gausskernel/optimizer/prep/prepnonjointree.cpp
@@ -1941,7 +1941,11 @@ static void reduce_orderby_recurse(Query* query, Node* jtnode, bool reduce)
         /* Reduce orderby clause in subquery for join or from clause of more than one rte */
         reduce_orderby_final(rte, reduce);
     } else if (IsA(jtnode, FromExpr)) {
-        /* If there is ROWNUM, can not reduce orderby clause in subquery */
+        /* If there is ROWNUM, can not reduce orderby clause in subquery from fromlist.
+         * For example, If there is a SQL {select * from table_name where rownum < n union
+         * select * from (select * from table_name order by column_name desc) where rownum < n;},
+         * can not reduce orderby clause here.
+         */
         if (contain_rownum_walker(jtnode, NULL)) {
             return;
         }
diff --git a/src/test/regress/expected/xc_rownum.out b/src/test/regress/expected/xc_rownum.out
index 331e8485b..b9267f09c 100644
--- a/src/test/regress/expected/xc_rownum.out
+++ b/src/test/regress/expected/xc_rownum.out
@@ -1640,5 +1640,15 @@ explain select id from student where rownum < 3 union select id from (select id
                      ->  Seq Scan on student  (cost=0.00..21.34 rows=1134 width=4)
 (9 rows)
 
+select * from test where id < 2 union select * from (select * from test order by id desc) where rownum < 5;
+ id | testchar 
+----+----------
+  8 | test8
+  7 | test7
+  9 | test9
+  1 | test1
+ 10 | test10
+(5 rows)
+
 drop table student;
 drop table test;
diff --git a/src/test/regress/sql/xc_rownum.sql b/src/test/regress/sql/xc_rownum.sql
index ef3b8ffe6..c0a40a971 100644
--- a/src/test/regress/sql/xc_rownum.sql
+++ b/src/test/regress/sql/xc_rownum.sql
@@ -460,6 +460,7 @@ explain select id from test where rownum < 5 group by id;
 
 -- ROWNUM with UNION and ORDER BY
 explain select id from student where rownum < 3 union select id from (select id from student order by 1)  where rownum < 5;
+select * from test where id < 2 union select * from (select * from test order by id desc) where rownum < 5;
 
 drop table student;
 drop table test;
\ No newline at end of file