修复了关于rownum的4个bug。

1.最大值只有255，正常应该是无限大。
2.group by跟where rownum < n同时存在时，禁止将rownum优化成limit
3.groupy by后面没有rownum，而having后面有rownum时，会正常运行，正常应该报错。
4.union前后的子句如果有where rownum < n，并且还有含有order by的子查询时，禁止将order by优化删除掉。

src/common/backend/parser/parse_agg.cpp

@@ -740,6 +740,26 @@ 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)));
+            }
+        }
+    }
     /*
      * If we have an ungrouped Var of the original query level, we have a
      * failure.  Vars below the original query level are not a problem, and

src/gausskernel/optimizer/prep/prepjointree.cpp

@@ -2963,8 +2963,8 @@ void preprocess_rownum(PlannerInfo *root, Query *parse)
     if (quals == NULL) {
         return;
     }
-    /* If it includes {order by}, can not be rewrited */
-    if (parse->sortClause != NULL) {
+    /* If it includes {order by} or {group by}, can not be rewrited */
+    if ((parse->sortClause != NULL) || (parse->groupClause != NULL)) {
         return;
     }
     if (parse->limitCount != NULL) {

src/gausskernel/optimizer/prep/prepnonjointree.cpp

@@ -1941,6 +1941,10 @@ 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 (contain_rownum_walker(jtnode, NULL)) {
+            return;
+        }
         FromExpr* f = (FromExpr*)jtnode;
         ListCell* l = NULL;
         bool flag = false;

src/gausskernel/runtime/executor/execQual.cpp

@@ -979,7 +979,7 @@ static Datum ExecEvalRownum(RownumState* exprstate, ExprContext* econtext, bool*
         *isDone = ExprSingleResult;
     *isNull = false;
 
-    return Int8GetDatum(exprstate->ps->ps_rownum + 1);
+    return Int64GetDatum(exprstate->ps->ps_rownum + 1);
 }
 
 /* ----------------------------------------------------------------

src/test/regress/expected/xc_rownum.out

@@ -241,7 +241,31 @@ select rownum + 1 rn from dual group by rn;
 ----
   2
 (1 row)
+
 
+--test having
+select id from distributors group by rownum,id having rownum < 5;
+ id 
+----
+  1
+  1
+  1
+  1
+(4 rows)
+
+select rownum from distributors group by rownum having rownum < 5;
+ rownum 
+--------
+      3
+      1
+      2
+      4
+(4 rows)
+
+select id from distributors group by id having rownum < 5;
+ERROR:  ROWNUM must appear in the GROUP BY clause or be used in an aggregate function
+LINE 1: select id from distributors group by id having rownum < 5;
+                                                       ^
 --test alias name after where
 select rownum rn, name from distributors where rn<3;
 ERROR:  Alias "rn" reference with ROWNUM included is invalid.
@@ -1645,5 +1669,32 @@ explain select * from test where rownum < 5 order by 1;
 (4 rows)
 
 
+-- ROWNUM with GROUP BY
+explain select id from test where rownum < 5 group by id;
+                         QUERY PLAN                          
+-------------------------------------------------------------
+ HashAggregate  (cost=25.12..25.79 rows=67 width=4)
+   Group By Key: id
+   ->  Seq Scan on test  (cost=0.00..24.18 rows=378 width=4)
+         Filter: (ROWNUM < 5)
+(4 rows)
+
+
+-- 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;
+                                    QUERY PLAN                                     
+-----------------------------------------------------------------------------------
+ HashAggregate  (cost=79.04..79.10 rows=6 width=4)
+   Group By Key: public.student.id
+   ->  Append  (cost=0.00..79.02 rows=6 width=4)
+         ->  Limit  (cost=0.00..0.04 rows=2 width=4)
+               ->  Seq Scan on student  (cost=0.00..21.34 rows=1134 width=4)
+         ->  Limit  (cost=78.87..78.92 rows=4 width=4)
+               ->  Sort  (cost=78.87..81.71 rows=1134 width=4)
+                     Sort Key: public.student.id
+                     ->  Seq Scan on student  (cost=0.00..21.34 rows=1134 width=4)
+(9 rows)
+
+
 drop table student;
 drop table test;

src/test/regress/sql/xc_rownum.sql

@@ -85,6 +85,11 @@ select rownum from distributors group by rownum;
 select rownum rn from distributors group by rn;
 select rownum + 1 from dual group by rownum;
 select rownum + 1 rn from dual group by rn;
+
+--test having
+select id from distributors group by rownum,id having rownum < 5;
+select rownum from distributors group by rownum having rownum < 5;
+select id from distributors group by id having rownum < 5;
 --test alias name after where
 select rownum rn, name from distributors where rn<3;
 select rownum rowno2, * from (select rownum rowno1, * from distributors order by id desc) where rowno2 < 2;
@@ -447,5 +452,11 @@ 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;
 
+-- ROWNUM with GROUP BY
+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;
+
 drop table student;
 drop table test;