Bug #30039 Add "Using group-by" to Extra column in EXPLAIN output

Description:
I read that there's an effort to add more information to EXPLAIN.  One thing I don't see in EXPLAIN is where a GROUP BY is being performed.  Though you can examine the query and see, you can't examine EXPLAIN alone and see.  Even when you have the query text, it's often not obvious at which stage MySQL really performs the GROUP BY.

There is one special case, "Using index for group-by" but it only applies in limited circumstances, and is really an indication of how an index is being used (loose index scan).  Its primary purpose isn't to show that there's a GROUP BY.  It can even show up in a non-grouped DISTINCT query

How to repeat:
Here are some examples:

mysql> explain select Host from mysql.user group by Host;
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | user  | range | NULL          | PRIMARY | 180     | NULL |    1 | Using index for group-by | 
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
1 row in set (0.01 sec)

mysql> explain select count(*) from mysql.user group by Host;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | user  | index | NULL          | PRIMARY | 228     | NULL |   10 | Using index | 
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select distinct Host from mysql.user;
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | user  | range | NULL          | PRIMARY | 180     | NULL |    1 | Using index for group-by | 
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+

Suggested fix:
I propose adding another value to the Extra column: "Using group-by."  I chose this wording because it seems consistent with "Using where."  In cases where there's a grouped JOIN and thus multiple rows in EXPLAIN, I am not sure which row it should appear in.  Probably the optimizer developers can make a better suggestion.

Thank you for a reasonable feature request.