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

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.

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.

Based on @Che code, you can also use triggers on INSERT and on UPDATE to perf2 in order to keep the value in stats table up to date in realtime.

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

Then:

CREATE TRIGGER `count_up` AFTER INSERT   ON `perf2` FOR EACH ROW UPDATE `stats`
SET   `stats`.`value` = `stats`.`value` + 1 
WHERE `stats`.`key` = 'perf2_count';

CREATE TRIGGER `count_down` AFTER DELETE ON `perf2` FOR EACH ROW UPDATE `stats`
SET   `stats`.`value` = `stats`.`value` - 1 
WHERE `stats`.`key` = 'perf2_count';

So the number of rows in the perf2 table can be read using this query, in realtime:

SELECT `value` FROM `stats` WHERE `key` = 'perf2_count';

This would have the advantage of eliminating the performance issue of performing a COUNT(*) and would only be executed when data changes in perf2.