mysql - How to optimize COUNT(*) performance on InnoDB by using index

Question

I have a largish but narrow InnoDB table with ~9m records. Doing count(*) or count(id) on the table is extremely slow (6+ seconds):

DROP TABLE IF EXISTS `perf2`;

CREATE TABLE `perf2` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `channel_id` int(11) DEFAULT NULL,
  `timestamp` bigint(20) NOT NULL,
  `value` double NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `ts_uniq` (`channel_id`,`timestamp`),
  KEY `IDX_CHANNEL_ID` (`channel_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

RESET QUERY CACHE;
SELECT COUNT(*) FROM perf2;

While the statement is not run too often it would be nice to optimize it. According to http://www.cloudspace.com/blog/2009/08/06/fast-mysql-innodb-count-really-fast/ this should be possible by forcing InnoDB to use an index:

SELECT COUNT(id) FROM perf2 USE INDEX (PRIMARY);

The explain plan seems fine:

id  select_type table   type    possible_keys   key     key_len ref     rows    Extra
1   SIMPLE      perf2   index   NULL            PRIMARY 4       NULL    8906459 Using index

Unfortunately the statement is as slow as before. According to "SELECT COUNT(*)" is slow, even with where clause I've also tried optimizing the table without success.

What/is the/re a way to optimize COUNT(*) performance on InnoDB?

Answer 1

As of MySQL 5.1.6 you can use the Event Scheduler and insert the count to a stats table regularly.

First create a table to hold the count:

CREATE TABLE stats (
`key` varchar(50) NOT NULL PRIMARY KEY,
`value` varchar(100) NOT NULL);

Then create an event to update the table:

CREATE EVENT update_stats
ON SCHEDULE
  EVERY 5 MINUTE
DO
  INSERT INTO stats (`key`, `value`)
  VALUES ('data_count', (select count(id) from data))
  ON DUPLICATE KEY UPDATE value=VALUES(value);

It's not perfect but it offers a self contained solution (no cronjob or queue) that can be easily tailored to run as often as the required freshness of the count.