Mysql Innodb "select count(*)" performance [duplicate] - mysql

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.

Answer 3

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.