Merge mysql.com:/home/ram/work/mysql-5.1-maint

into  mysql.com:/home/ram/work/b31438/b31438.5.1

mysql-test/r/type_datetime.result

@@ -436,6 +436,20 @@ f1
 Warnings:
 Warning	1292	Incorrect datetime value: '0000-00-00 45:44:44'
 drop table t1;
+create table t1 (a tinyint);
+insert into t1 values (), (), ();
+select sum(a) from t1 group by convert(a, datetime);
+sum(a)
+NULL
+select convert(a, datetime) from t1;
+Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
+def					convert(a, datetime)	12	29	0	Y	128	6	63
+convert(a, datetime)
+NULL
+NULL
+NULL
+drop table t1;
+End of 5.0 tests
 set @org_mode=@@sql_mode;
 create table t1 (da date default '1962-03-03 23:33:34', dt datetime default '1962-03-03');
 Warnings:

mysql-test/t/type_datetime.test

@@ -293,6 +293,19 @@ insert into t1 set f1 = '15:44:44';
 select * from t1 where (convert(f1,datetime)) != 1;
 drop table t1;
 
+#
+# Bug #31249: problem with convert(..., datetime)
+#
+create table t1 (a tinyint);
+insert into t1 values (), (), ();
+select sum(a) from t1 group by convert(a, datetime);
+--enable_metadata
+select convert(a, datetime) from t1;
+--disable_metadata
+drop table t1;
+
+--echo End of 5.0 tests
+
 #
 # Test of storing datetime into date fields
 #

sql/item_timefunc.h

@@ -847,7 +847,9 @@ public:
   }
   void fix_length_and_dec()
   {
-    Item_typecast_maybe_null::fix_length_and_dec();
+    collation.set(&my_charset_bin);
+    maybe_null= 1;
+    max_length= MAX_DATETIME_FULL_WIDTH * MY_CHARSET_BIN_MB_MAXLEN;
     decimals= DATETIME_DEC;
   }
   bool result_as_longlong() { return TRUE; }

sql/sql_select.cc

@@ -14370,13 +14370,31 @@ calc_group_buffer(JOIN *join,ORDER *group)
                                                 group_item->decimals);
         break;
       case STRING_RESULT:
+      {
+        enum enum_field_types type= group_item->field_type();
         /*
-          Group strings are taken as varstrings and require an length field.
-          A field is not yet created by create_tmp_field()
-          and the sizes should match up.
+          As items represented as DATE/TIME fields in the group buffer
+          have STRING_RESULT result type, we increase the length 
+          by 8 as maximum pack length of such fields.
         */
-        key_length+= group_item->max_length + HA_KEY_BLOB_LENGTH;
+        if (type == MYSQL_TYPE_TIME ||
+            type == MYSQL_TYPE_DATE ||
+            type == MYSQL_TYPE_DATETIME ||
+            type == MYSQL_TYPE_TIMESTAMP)
+        {
+          key_length+= 8;
+        }
+        else
+        {
+          /*
+            Group strings are taken as varstrings and require an length field.
+            A field is not yet created by create_tmp_field()
+            and the sizes should match up.
+          */
+          key_length+= group_item->max_length + HA_KEY_BLOB_LENGTH;
+        }
         break;
+      }
       default:
         /* This case should never be choosen */
         DBUG_ASSERT(0);