Why is MySQL not using this index with a GROUP BY query? - mysql

Question

I have this big table (ca million records) and I'm trying to retrieve the last record of each type.
The table, the index and the query are very simple, and the fact that MySQL is not using the index means I must be overlooking something.
The table looks like this:
CREATE TABLE `MyTable001` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`TypeField` int(11) NOT NULL,
`Value` bigint(20) NOT NULL,
`Timestamp` bigint(20) NOT NULL,
`AnotherField1` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `idx_MyTable001_TypeField` (`TypeField`),
KEY `idx_MyTable001_Timestamp` (`Timestamp`)
) ENGINE=MyISAM
Show Index gives this:
+------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| MyTable001 | 0 | PRIMARY | 1 | id | A | 626141 | NULL | NULL | | BTREE | | |
| MyTable001 | 1 | idx_MyTable001_TypeField | 1 | TypeField | A | 458 | NULL | NULL | | BTREE | | |
| MyTable001 | 1 | idx_MyTable001_Timestamp | 1 | Timestamp | A | 156535 | NULL | NULL | | BTREE | | |
+------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
But when I execute EXPLAIN for the following query:
SELECT *
FROM MyTable001
GROUP BY TypeField
ORDER BY id DESC
The result is this:
+----+-------------+------------+------+---------------+------+---------+------+--------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+---------------+------+---------+------+--------+---------------------------------+
| 1 | SIMPLE | MyTable001 | ALL | NULL | NULL | NULL | NULL | 626141 | Using temporary; Using filesort |
+----+-------------+------------+------+---------------+------+---------+------+--------+---------------------------------+
Why won't MySQL use idx_MyTable001_TypeField?
Thanks in advance.

Answer 1

The problem is that the content of the fields not in the group by are still being inspected. Therefore, all rows must be read, and it's better to do a full table scan. This is clearly seen with the following examples:
SELECT TypeField, COUNT(*) FROM MyTable001 GROUP BY TypeField uses the index.
SELECT TypeField, COUNT(id) FROM MyTable001 GROUP BY TypeField does not.
The original query was incorrect. The correct query is:
SELECT l.*
FROM MyTable001 l
JOIN (
SELECT MAX(id) m_id
FROM MyTable001 l
GROUP BY l.TypeField) l_id ON l_id.m_id = l.id;
It takes 260ms in a table with 630k records. Joachim Isaksson's and fancyPants' alternatives took several minutes in my tests.