MySQL not using multiple column index for some queries

Question

I have a table with several million records on MySQL in an MyISAM table. Very simplified, it's like this:

CREATE TABLE `test` (
  `text` varchar(5) DEFAULT NULL,
  `number` int(5) DEFAULT NULL,
  KEY `number` (`number`) USING BTREE,
  KEY `text_number` (`text`,`number`) USING BTREE
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

It's populated with this data:

INSERT INTO `test` VALUES ('abcd', '1');
INSERT INTO `test` VALUES ('abcd', '2');
INSERT INTO `test` VALUES ('abcd', '3');
INSERT INTO `test` VALUES ('abcd', '4');
INSERT INTO `test` VALUES ('bbbb', '1');
INSERT INTO `test` VALUES ('bbbb', '2');
INSERT INTO `test` VALUES ('bbbb', '3');

When I run the following query:

EXPLAIN SELECT * FROM `test` WHERE (`text` = 'bbbb' AND `number` = 2)

It returns 'number' as the key to use. But the following query:

EXPLAIN SELECT * FROM `test` WHERE (`text` = 'bbbb' AND `number` = 1)

Returns 'text_number' as key to use, which would make more sense to me as this combined key matches exactly with the 2 columns in the WHERE. On these amount of records the performance isn't an issue, but on several million records the query which uses the 'text' index takes 4 seconds, and the one that uses 'text_number' index is finished in several milliseconds.

Is there a logical explaination for this? How can I change the index that MySQL uses the index? I know I can use USE INDEX but I want MySQL to be able to find the best plan to execute the query. This is on MySQL 5.1 and 5.5, same results.

Answer 1

If you expect some of the queries to be game changer for the performance it is good idea to use you own indexes, like:

EXPLAIN SELECT * FROM `test` USE INDEX (number) WHERE (`text` = 'bbbb' AND `number` = 1);
or
EXPLAIN SELECT * FROM `test` USE INDEX (text_number) WHERE (`text` = 'bbbb' AND `number` = 1);

Generally, you can relay on the built-in query optimizer for most of the queries, but for the crucial or problematic ones it is good idea to look closer.

Answer 2

Two things that could improve your performance:

1. ANALYZE TABLE to update your optimizer's index statistics so it knows the current key distribution. You can control the optimizer's statistics generation in a number of ways.
2. Create a second covering index with the fields reversed. In the real world, this involves figuring out the optimal one or two covering indexes for a set of three or four columns, and that's something you arrive at through benchmarking and EXPLAIN analysis. Have a look at this answer for some advice on doing that.