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MySQL COUNT(*)性能原理详解_Mysql_
2023-05-26
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简介 MySQL COUNT(*)性能原理详解_Mysql_
前言
在实际开发过程中，统计一个表的数据量是经常遇到的需求，用来统计数据库表的行数都会使用COUNT(*)，COUNT(1)或者COUNT(字段)，但是表中的记录越来越多，使用COUNT(*)也会变得越来越慢，今天我们就来分析一下COUNT(*)的性能到底如何。
1.COUNT(1)、COUNT(*)与COUNT(字段)哪个更快？
执行效果：
COUNT(*)MySQL 对count(*)进行了优化，count(*)直接扫描主键索引记录，并不会把全部字段取出来，直接按行累加。COUNT(1)InnoDB引擎遍历整张表，但不取值，server 层对于返回的每一行，放一个数字“1”进去，按行累加。COUNT(字段)如果这个“字段”是定义为NOT NULL，那么InnoDB 引擎会一行行地从记录里面读出这个字段，server 层判断不能为NULL，按行累加;如果这个“字段”定义允许为NULL，那么InnoDB 引擎会一行行地从记录里面读出这个字段，然后把值取出来再判断一下，不是 NULL才累加。
实验分析
本文测试使用的环境：
[root@zhyno1 ~]# cat /etc/system-release CentOS Linux release 7.9.2009 (Core) [root@zhyno1 ~]# uname -a Linux zhyno1 3.10.0-1160.62.1.el7.x86_64 #1 SMP Tue Apr 5 16:57:59 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
测试数据库采用的是（存储引擎采用InnoDB，其它参数默认）：
(Mon Jul 25 09:41:39 2022)[root@GreatSQL][(none)]>select version(); +-----------+ | version() | +-----------+ | 8.0.25-16 | +-----------+ 1 row in set (0.00 sec)
实验开始：
#首先我们创建一个实验表 CREATE TABLE test_count ( `id` int(10) NOT NULL AUTO_INCREMENT PRIMARY KEY, `name` varchar(20) NOT NULL, `salary` int(1) NOT NULL, KEY `idx_salary` (`salary`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; #插入1000W条数据 DELIMITER // CREATE PROCEDURE insert_1000w() BEGIN DECLARE i INT; SET i=1; WHILE i<=10000000 DO INSERT INTO test_count(name,salary) VALUES('KAiTO',1); SET i=i+1; END WHILE; END// DELIMITER ; #执行存储过程 call insert_1000w();接下来我们分别来实验一下：
COUNT(1)花费了4.19秒
(Sat Jul 23 22:56:04 2022)[root@GreatSQL][test]>select count(1) from test_count; +----------+ | count(1) | +----------+ | 10000000 | +----------+ 1 row in set (4.19 sec)
COUNT(*)花费了4.16秒
(Sat Jul 23 22:57:41 2022)[root@GreatSQL][test]>select count(*) from test_count; +----------+ | count(*) | +----------+ | 10000000 | +----------+ 1 row in set (4.16 sec)
COUNT(字段)花费了4.23秒
(Sat Jul 23 22:58:56 2022)[root@GreatSQL][test]>select count(id) from test_count; +-----------+ | count(id) | +-----------+ | 10000000 | +-----------+ 1 row in set (4.23 sec)
我们可以再来测试一下执行计划
COUNT(*)
(Sat Jul 23 22:59:16 2022)[root@GreatSQL][test]>explain select count(*) from test_count; +----+-------------+------------+------------+-------+---------------+------------+---------+------+---------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+-------+---------------+------------+---------+------+---------+----------+-------------+ | 1 | SIMPLE | test_count | NULL | index | NULL | idx_salary | 4 | NULL | 9980612 | 100.00 | Using index | +----+-------------+------------+------------+-------+---------------+------------+---------+------+---------+----------+-------------+ 1 row in set, 1 warning (0.01 sec) (Sat Jul 23 22:59:48 2022)[root@GreatSQL][test]>show warnings; +-------+------+-----------------------------------------------------------------------+ | Level | Code | Message | +-------+------+-----------------------------------------------------------------------+ | Note | 1003 | /* select#1 */ select count(0) AS `count(*)` from `test`.`test_count` | +-------+------+-----------------------------------------------------------------------+ 1 row in set (0.00 sec)
COUNT(1)
(Sat Jul 23 23:12:45 2022)[root@GreatSQL][test]>explain select count(1) from test_count; +----+-------------+------------+------------+-------+---------------+------------+---------+------+---------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+-------+---------------+------------+---------+------+---------+----------+-------------+ | 1 | SIMPLE | test_count | NULL | index | NULL | idx_salary | 4 | NULL | 9980612 | 100.00 | Using index | +----+-------------+------------+------------+-------+---------------+------------+---------+------+---------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) (Sat Jul 23 23:13:02 2022)[root@GreatSQL][test]>show warnings; +-------+------+-----------------------------------------------------------------------+ | Level | Code | Message | +-------+------+-----------------------------------------------------------------------+ | Note | 1003 | /* select#1 */ select count(1) AS `count(1)` from `test`.`test_count` | +-------+------+-----------------------------------------------------------------------+ 1 row in set (0.00 sec)
COUNT(字段)
(Sat Jul 23 23:13:14 2022)[root@GreatSQL][test]>explain select count(id) from test_count; +----+-------------+------------+------------+-------+---------------+------------+---------+------+---------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+-------+---------------+------------+---------+------+---------+----------+-------------+ | 1 | SIMPLE | test_count | NULL | index | NULL | idx_salary | 4 | NULL | 9980612 | 100.00 | Using index | +----+-------------+------------+------------+-------+---------------+------------+---------+------+---------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) (Sat Jul 23 23:13:29 2022)[root@GreatSQL][test]>show warnings; +-------+------+-----------------------------------------------------------------------------------------------+ | Level | Code | Message | +-------+------+-----------------------------------------------------------------------------------------------+ | Note | 1003 | /* select#1 */ select count(`test`.`test_count`.`id`) AS `count(id)` from `test`.`test_count` | +-------+------+-----------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)
需要注意的是COUNT里如果是非主键字段的话
(Tue Jul 26 14:01:57 2022)[root@GreatSQL][test]>explain select count(name) from test_count where id <100 ; +----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1 | SIMPLE | test_count | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 99 | 100.00 | Using where | +----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
实验结果
- 1.从上面的实验我们可以得出，
COUNT(*)和COUNT(1)是最快的，其次是COUNT(id)。 - 2.
count(*)被MySQL查询优化器改写成了count(0)，并选择了idx_salary索引。 - 3.
count(1)和count(id)都选择了idx_salary索引。
实验结论
总结：COUNT(*)=COUNT(1)>COUNT(id)
MySQL的官方文档也有说过：
InnoDB handles SELECT COUNT(*) and SELECT COUNT(1) operations in the same way. There is no performance difference
翻译： InnoDB以相同的方式处理SELECT COUNT（*）和SELECT COUNT（1）操作。没有性能差异
所以说明了对于COUNT(1)或者是COUNT(*)，MySQL的优化其实是完全一样的，没有存在没有性能的差异。
但是建议使用COUNT(*)，因为这是MySQL92定义的标准统计行数的语法。
2.COUNT(*)与TABLES_ROWS
在InnoDB中，MySQL数据库每个表占用的空间、表记录的行数可以打开MySQL的information_schema数据库。在该库中有一个TABLES表，这个表主要字段分别是：
- TABLE_SCHEMA : 数据库名
- TABLE_NAME：表名
- ENGINE：所使用的存储引擎
- TABLES_ROWS：记录数
- DATA_LENGTH：数据大小
- INDEX_LENGTH：索引大小
TABLE_ROWS用于显示这个表当前有多少行，这个命令执行挺快的，那这个TABLE_ROWS能代替count(*)吗？
我们用TABLES_ROWS查询一下表记录条数：
(Sat Jul 23 23:15:14 2022)[root@GreatSQL][test]>SELECT TABLE_ROWS -> FROM INFORMATION_SCHEMA.TABLES -> WHERE TABLE_NAME = 'test_count'; +------------+ | TABLE_ROWS | +------------+ | 9980612 | +------------+ 1 row in set (0.03 sec)
可以看到，记录的条数并不准确，因为InnoDB引擎下TABLES_ROWS行计数仅是大概估计值。
