索引最佳实践
示例:
CREATE TABLE `employees` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(24) NOT NULL DEFAULT '' COMMENT '姓名',
`age` int(11) NOT NULL DEFAULT '0' COMMENT '年龄',
`position` varchar(20) NOT NULL DEFAULT '' COMMENT '职位',
`hire_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入职时间',
PRIMARY KEY (`id`),
KEY `idx_name_age_position` (`name`,`age`,`position`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 COMMENT='员工记录表';
INSERT INTO employees(name,age,position,hire_time) VALUES('LiLei',22,'manager',NOW());
INSERT INTO employees(name,age,position,hire_time) VALUES('HanMeimei', 23,'dev',NOW());
INSERT INTO employees(name,age,position,hire_time) VALUES('Lucy',23,'dev',NOW());
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# 1.全值匹配
EXPLAIN SELECT * FROM employees WHERE name= 'LiLei';
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如果用到联合索引,则要遵守最左侧前缀原则。指的是查询从索引的最左前列开始并且不跳过索引中的列。
EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 22;
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EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 22 AND position ='manage r';
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# 2.最左前缀法则
如果索引了多列,要遵守最左前缀法则。指的是查询从索引的最左前列开始并且不跳过索引中的列。
EXPLAIN SELECT * FROM employees WHERE name = 'Bill' and age = 31;
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EXPLAIN SELECT * FROM employees WHERE age = 30 AND position = 'dev';
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# 3.不在索引列上做任何操作(计算、函数、(自动or手动)类型转换),会导致索引失效而转向全表扫描
EXPLAIN SELECT * FROM employees WHERE left(name,3) = 'LiLei';
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ALTER TABLE `employees` ADD INDEX `idx_hire_time` (`hire_time`) USING BTREE ;
EXPLAIN select * from employees where date(hire_time) ='2018‐09‐30';
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转化为日期范围查询,有可能会走索引:
EXPLAIN select * from employees where hire_time >='2018‐09‐30 00:00:00.0' and hire_time <='2021‐09‐30 23:59:59.0';
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# 4.存储引擎不能使用索引中范围条件右边的列
EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 22 AND position ='manage r';
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EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age > 22 AND position ='manage r';
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# 5.尽量使用覆盖索引(只访问索引的查询(索引列包含查询列))
减少 select * 语句,只访问二级索引。
EXPLAIN SELECT name,age FROM employees WHERE name= 'LiLei' AND age = 23 AND position ='manager';
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EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 23 AND position ='manager';
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# 6.mysql在使用不等于(!=或<>),not in ,not exists 的时候无法使用索引会导致全表扫描 < 小于、 > 大于、 <=、>= 这些,mysql内部优化器会根据检索比例、表大小等多个因素整体评估是否使用索引
# 7.is null,is not null 一般情况下也无法使用索引
EXPLAIN SELECT * FROM employees WHERE name is null
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# 8.like以通配符开头('$abc...')mysql索引失效会变成全表扫描操作
EXPLAIN SELECT * FROM employees WHERE name like '%Lei'
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EXPLAIN SELECT * FROM employees WHERE name like 'Lei%'
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# 问题:解决like'%字符串%'索引不被使用的方法?
使用覆盖索引,查询字段必须是建立覆盖索引字段
EXPLAIN SELECT name,age,position FROM employees WHERE name like '%Lei%';
1如果不能使用覆盖索引则可能需要借助搜索引擎(如ES等)
# 9.字符串不加单引号索引失效
EXPLAIN SELECT * FROM employees WHERE name = '1000';
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# 10.少用or或in,用它查询时,mysql不一定使用索引,mysql内部优化器会根据检索比例、表大小等多个因素整体评 估是否使用索引,详见范围查询优化
EXPLAIN SELECT * FROM employees WHERE name = 'LiLei' or name = 'HanMeimei';
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# 11.范围查询优化
在age列上增加索引
ALTER TABLE `employees` ADD INDEX `idx_age` (`age`) USING BTREE ;
explain select * from employees where age >=1 and age <=2000;
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没走索引原因:mysql内部优化器会根据检索比例、表大小等多个因素整体评估是否使用索引。比如这个例子,可能是 由于单次数据量查询过大导致优化器最终选择不走索引
优化方法:可以将大的范围拆分成多个小范围
explain select * from employees where age >=1 and age <=200;
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上次更新: 2023/05/09, 17:54:37