A fix (bug #5615 type of aggregate function column wrong when using group by).

mysql-test/r/func_group.result:
  test case (bug #5615 type of aggregate function column wrong when using group by)
mysql-test/t/func_group.test:
  test case (bug #5615 type of aggregate function column wrong when using group by)
sql/sql_select.cc:
  A fix (bug #5615 type of aggregate function column wrong when using group by):
    should create a temporary field of the proper type in case of 
    MIN|MAX(field).

mysql-test/r/func_group.result

@@ -719,3 +719,12 @@ one	2
 two	2
 three	1
 drop table t1;
+create table t1(a int, b datetime);
+insert into t1 values (1, NOW()), (2, NOW());
+create table t2 select MAX(b) from t1 group by a;
+show create table t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `MAX(b)` datetime default NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+drop table t1, t2;

mysql-test/t/func_group.test

@@ -458,3 +458,13 @@ INSERT INTO t1 VALUES
 
 select val, count(*) from t1 group by val;
 drop table t1;
+
+#
+# Bug #5615: type of aggregate function column wrong when using group by
+#
+
+create table t1(a int, b datetime);
+insert into t1 values (1, NOW()), (2, NOW());
+create table t2 select MAX(b) from t1 group by a;
+show create table t2;
+drop table t1, t2;

sql/sql_select.cc

@@ -4744,6 +4744,15 @@ Field *create_tmp_field(THD *thd, TABLE *table,Item *item, Item::Type type,
 				item->name,table,item_sum->decimals);
     case Item_sum::UNIQUE_USERS_FUNC:
       return new Field_long(9,maybe_null,item->name,table,1);
+    case Item_sum::MIN_FUNC:
+    case Item_sum::MAX_FUNC:
+      if (item_sum->args[0]->type() == Item::FIELD_ITEM)
+      {
+        *from_field= ((Item_field*) item_sum->args[0])->field;
+        return create_tmp_field_from_field(thd, *from_field, item, table,
+                                           modify_item, convert_blob_length);
+      }
+      /* fall through */
     default:
       switch (item_sum->result_type()) {
       case REAL_RESULT: