Fix bug#12887 Distinct is not always applied after rollup

For queries with GROUP BY and without hidden GROUP BY fields DISTINCT is
optimized away becuase such queries produce result set without duplicates.
But ROLLUP can add rows which may be same to some rows and this fact was
ignored.

Added check so if ROLLUP is present DISTINCT can't be optimized away.

mysql-test/r/olap.result

@@ -529,3 +529,15 @@ a	LENGTH(a)	COUNT(*)
 2	1	1
 NULL	NULL	2
 DROP TABLE t1;
+create table t1 ( a varchar(9), b int );
+insert into t1 values('a',1),(null,2);
+select a, max(b) from t1 group by a with rollup;
+a	max(b)
+NULL	2
+a	1
+NULL	2
+select distinct a, max(b) from t1 group by a with rollup;
+a	max(b)
+NULL	2
+a	1
+drop table t1;

mysql-test/t/olap.test

@@ -263,4 +263,13 @@ SELECT * FROM (SELECT a, LENGTH(a), COUNT(*) FROM t1 GROUP BY a WITH ROLLUP) t;
 
 DROP TABLE t1;
 
+#
+# Bug #12887 Distinct is not always applied after rollup
+#
+create table t1 ( a varchar(9), b int );
+insert into t1 values('a',1),(null,2);
+select a, max(b) from t1 group by a with rollup;
+select distinct a, max(b) from t1 group by a with rollup;
+drop table t1;
+
 # End of 4.1 tests

sql/sql_select.cc

@@ -618,7 +618,7 @@ JOIN::optimize()
   }
   if (group_list || tmp_table_param.sum_func_count)
   {
-    if (! hidden_group_fields)
+    if (! hidden_group_fields && rollup.state == ROLLUP::STATE_NONE)
       select_distinct=0;
   }
   else if (select_distinct && tables - const_tables == 1)