This MySQL query doesn't use my indexes - mysql

Question

The following query get a lists of computer groups. For each group it sums how many computers have state=1 and how many have state=2
SELECT
cg.id
cg.order
cg.group_mode
cg.created
cg.updated
SUM(
CASE WHEN c.state = 1 THEN 1 ELSE 0 END
) AS sclr7,
SUM(
CASE WHEN c.state = 2 THEN 1 ELSE 0 END
) AS sclr8,
FROM
computer_group cg
LEFT JOIN computer c ON cg.id = c.group_id
WHERE
cg.group_mode <> 3
GROUP BY
cg.id
ORDER BY
cg.order ASC;
When i run EXPLAIN on this query, mysql returns: Using where; Using temporary; Using filesort
+----+-------------+-------+------+----------------------+----------------------+---------+--------------------+------+----------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+----------------------+----------------------+---------+--------------------+------+----------+----------------------------------------------+
| 1 | SIMPLE | cg | ALL | mode | NULL | NULL | NULL | 33 | 100.00 | Using where; Using temporary; Using filesort |
| 1 | SIMPLE | c | ref | IDX_39B3258BBE45D62E | IDX_39B3258BBE45D62E | 768 | test.c.id | 57 | 100.00 | |
+----+-------------+-------+------+----------------------+----------------------+---------+--------------------+------+----------+----------------------------------------------+
I have the following indexes:
computer_group table
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| computer_group | 0 | PRIMARY | 1 | id | A | 33 | NULL | NULL | | BTREE | |
| computer_group | 1 | mode | 1 | mode | A | 3 | NULL | NULL | | BTREE | |
| computer_group | 1 | order | 1 | order | A | 33 | NULL | NULL | | BTREE | |
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
(something went wrong with copy/paste the computer_group, it is now fixed)
computer table
+--------------+------------+----------------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+--------------+------------+----------------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+
| computer | 0 | PRIMARY | 1 | id | A | 2611 | NULL | NULL | | BTREE | |
| computer | 1 | IDX_39B3258BBE45D62E | 1 | group_id | A | 32 | NULL | NULL | YES | BTREE | |
| computer | 1 | state | 1 | state | A | 1 | NULL | NULL | | BTREE | |
+--------------+------------+----------------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+
I tried to add various indexes but it seems i cant get to prevent the filesort and the temporary table.
This is driving me nuts. I have spend several days trying to fixing this. Am i doing something wrong or is this not preventable?

Answer 1

You should be able to get it to use indexes if you eliminate the outer group by:
SELECT cg.*, c.sclr7, c.sclr8
FROM computer_group cg JOIN
(SELECT c.group_id, SUM(c.state = 1) as sclr7, SUM(c.state = 1) on sclr8
FROM computer c
) c
ON cg.id = c.group_id
WHERE cg.group_mode <> 3
ORDER BY cg.order ASC;
MySQL might use an index on computer_group(order, group_mode) for the query.
The join might actually confuse MySQL. A surer query is this:
SELECT cg.*,
(SELECT SUM(c.state = 1)
FROM computer c
WHERE cg.id = c.group_id
) as sclr7,
(SELECT SUM(c.state = 2)
FROM computer c
WHERE cg.id = c.group_id
) as sclr8
FROM computer_group cg
ON cg.id = c.group_id
WHERE cg.group_mode <> 3
ORDER BY cg.order ASC;
You want an index on computer_group(order, group_mode, id) and computer(group_id, state).