ORDER BY column from the joined table performance - mysql

Question

How can I improve this? It takes around half a second and it's just a demo query.
The problem here is the ORDER BY, but I can't really do without it. I also need empty rows of the LEFT JOIN for missing records in something table.
SELECT c.name
FROM customers c
LEFT JOIN something s USING(customer_id)
ORDER BY s.test DESC LIMIT 25
DB schema:
CREATE TABLE customers (
customer_id int(11) NOT NULL AUTO_INCREMENT,
name text NOT NULL,
PRIMARY KEY (customer_id),
KEY namne (name(999))
) ENGINE=MyISAM AUTO_INCREMENT=100001 DEFAULT CHARSET=latin
CREATE TABLE something (
id int(11) NOT NULL AUTO_INCREMENT,
customer_id int(11) NOT NULL,
text longtext NOT NULL,
test varchar(5) NOT NULL,
PRIMARY KEY (id),
KEY customer_id (customer_id),
KEY text (text(999)),
KEY test (test),
KEY asdasd (customer_id,test)
) ENGINE=MyISAM AUTO_INCREMENT=12 DEFAULT CHARSET=latin1
EXPLAIN:
+------+-------------+-------+------+--------------------+--------+---------+--------------------+--------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+------+--------------------+--------+---------+--------------------+--------+---------------------------------+
| 1 | SIMPLE | c | ALL | NULL | NULL | NULL | NULL | 100000 | Using temporary; Using filesort |
| 1 | SIMPLE | s | ref | customer_id,asdasd | asdasd | 4 | test.c.customer_id | 2 | Using index |
+------+-------------+-------+------+--------------------+--------+---------+--------------------+--------+---------------------------------+

Answer 1

It doesn't look like the LEFT JOIN makes sence here. If you replace it by an INNER JOIN, the engine would be able to use the KEY test (test) for the ORDER BY clause. So all you need might be this:
SELECT c.name
FROM customers c
INNER JOIN something s USING(customer_id)
ORDER BY s.test DESC LIMIT 25
But to get the exactly same result as with the LEFT JOIN you can combine two fast queries with UNION ALL:
(
SELECT c.name
FROM customers c
LEFT JOIN something s USING(customer_id)
ORDER BY s.test DESC LIMIT 25
) UNION ALL (
SELECT c.name
FROM customers c
LEFT JOIN something s USING(customer_id)
WHERE s.customer_id IS NULL
LIMIT 25
)
LIMIT 25

Answer 2

Use InnoDB, not MyISAM.
Use some sensible limit in a VARCHAR(..) instead of TEXT.
Then get rid of "prefix indexing" if possible.
INDEX(a) is redundant when you have INDEX(a,b).
For more speed:
SELECT name
FROM (
( SELECT c.name, s.test
FROM customers c
JOIN something s USING(customer_id)
ORDER BY s.test DESC
LIMIT 25
)
UNION ALL
( SELECT c.name, NULL
FROM customers c
LEFT JOIN something s USING(customer_id)
WHERE s.test IS NULL
LIMIT 25
)
) AS x
ORDER BY test DESC
LIMIT 25
And have
INDEX(test, customer_id)