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.

Answer 2

For the time being I've solved the problem by using this approximation:

EXPLAIN SELECT COUNT(id) FROM data USE INDEX (PRIMARY)

The approximate number of rows can be read from the rows column of the explain plan when using InnoDB as shown above. When using MyISAM this will remain EMPTY as the table reference isbeing optimized away- so if empty fallback to traditional SELECT COUNT instead.