mysql 索引优化法则
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建表语句
CREATE TABLE staffs(
id INT PRIMARY KEY AUTO_INCREMENT,
NAME VARCHAR (24) NOT NULL DEFAULT '' COMMENT '姓名',
age INT NOT NULL DEFAULT 0 COMMENT '年龄',
pos VARCHAR (20) NOT NULL DEFAULT 0 COMMENT '职位',
add_time TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入职时间'
) CHARSET utf8 COMMENT '员工记录表';
INSERT INTO staffs(NAME, age, pos, add_time) VALUES('ronnie', 22, 'manager', NOW());
INSERT INTO staffs(NAME, age, pos, add_time) VALUES('John', 23, 'devloper', NOW());
INSERT INTO staffs(NAME, age, pos, add_time) VALUES('2181', 27, 'devloper', NOW());
-- 建复合索引
alter table staffs add index idx_staffs_nameAgePos(name, age, pos);mysql 优化法则
全值匹配我最爱(Kappa)
遵循最佳左前缀法则
如果索引了多列, 查询从索引的最左前列开始并且不跳过索引中的列。
其实索引本质是个单向链表, 你要先得到头才能逐渐往后取后面的, 中间断了就走不了后面的索引了。
正确使用 Demo:
mysql> explain select * from staffs where name = 'ronnie'; +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | 100.00 | NULL | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from staffs where name = 'ronnie' and age = 22; +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+ | 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 78 | const,const | 1 | 100.00 | NULL | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from staffs where name = 'ronnie' and age = 22 and pos = 'manager'; +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+ | 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 140 | const,const,const | 1 | 100.00 | NULL | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)违反使用Demo:(完全失效)[带头大哥死了]:
mysql> explain select * from staffs where age = 22 and pos = 'manager'; +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from staffs where age = 22; +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from staffs where pos = 'manager'; +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)- 完全失效, 执行了全表扫描。
违反使用Demo(部分失效)[中间兄弟断了]:
mysql> explain select * from staffs where name = 'ronnie' and pos = 'manager'; +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | 33.33 | Using index condition | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec)- 使用了复合索引 idx_staffs_nameAgePos, 但 reference 只有一个常量参数, 只用了最左侧的 name, 没用到 pos。
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
索引列上执行函数Demo:
mysql> explain select * from staffs where left(name,4) = 'John'; +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | Using where | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)- 可以看到执行了全表扫描, 索引失效。
索引列上执行计算Demo:
mysql> explain select * from staffs where char_length('name') > 3; +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | NULL | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)- 可以看到执行了全表扫描, 索引失效。
索引列上执行类型转换Demo:
mysql> select * from staffs where name = 2181; +----+------+-----+----------+---------------------+ | id | NAME | age | pos | add_time | +----+------+-----+----------+---------------------+ | 3 | 2181 | 27 | devloper | 2020-01-10 11:38:08 | +----+------+-----+----------+---------------------+ 1 row in set, 2 warnings (0.00 sec) mysql> explain select * from staffs where name = 2181; +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | idx_staffs_nameAgePos | NULL | NULL | NULL | 3 | 33.33 | Using where | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ 1 row in set, 3 warnings (0.00 sec) mysql> explain select * from staffs where name = '2181'; +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | 100.00 | NULL | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)- 这种就是比较坑的, 平时写sql是不注意, varChar类型请务必加上‘ ‘, 如果不加, mysql底层会进行类型的隐式转换, 造成索引失效。
存储引擎不能使用索引中范围条件右边的列
范围之后全失效, 所以建索引是一门学问, 需要对常用的业务sql进行综合考量。
Demo:
mysql> explain select * from staffs where name = 'John' and age > 22 and pos = 'devloper'; +----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | staffs | NULL | range | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 78 | NULL | 1 | 33.33 | Using index condition | +----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec)我们的索引是这样建的: name -> age -> pos
由于中间的age进行了范围查询, 会导致后续的索引全部失效。
尽量使用覆盖索引(只访问索引的查询(索引列和查询列一致)), 减少select *
对比Demo:
mysql> explain select * from staffs where name = 'John' and pos = 'devloper'; +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | 33.33 | Using index condition | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select name, age, pos from staffs where name = 'John' and pos = 'devloper'; +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+--------------------------+ | 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | 33.33 | Using where; Using index | +----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+--------------------------+ 1 row in set, 1 warning (0.00 sec)- 可以看到extra多了 Using index, 表面select操作中使用了覆盖索引(Covering Index), 避免了访问表的数据行。
mysql 在使用不等于(!= 或者 <>) 的时候无法使用索引会导致全表扫描
Demo:
mysql> explain select * from staffs where name != 'John'; +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | idx_staffs_nameAgePos | NULL | NULL | NULL | 3 | 66.67 | Using where | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from staffs where name <> 'John'; +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | idx_staffs_nameAgePos | NULL | NULL | NULL | 3 | 66.67 | Using where | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
is null, is not null 也无法使用索引
底层原因是: null 不能转换为索引表示的数字来索引
Demo:
mysql> explain select * from staffs where name is null; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from staffs where name is not null; +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | idx_staffs_nameAgePos | NULL | NULL | NULL | 3 | 66.67 | Using where | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
like 以通配符开头(‘%abc...‘) mysql 索引失效会变成全表扫描的操作
Demo:
mysql> explain select * from staffs where name like '%ronnie'; +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from staffs where name like '%ronnie%'; +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where | +----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from staffs where name like 'ronnie%'; +----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | staffs | NULL | range | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | NULL | 1 | 100.00 | Using index condition | +----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec)可以看到只有% 在右边的查询使用了索引
这个情况就比较蛋疼, 如果你们业务上就需要百分号在左边, 需要使用覆盖索引来优化。
mysql> explain select id, name from staffs where name like '%ronnie%'; +----+-------------+--------+------------+-------+---------------+-----------------------+---------+------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+-------+---------------+-----------------------+---------+------+------+----------+--------------------------+ | 1 | SIMPLE | staffs | NULL | index | NULL | idx_staffs_nameAgePos | 140 | NULL | 3 | 33.33 | Using where; Using index | +----+-------------+--------+------------+-------+---------------+-----------------------+---------+------+------+----------+--------------------------+ 1 row in set, 1 warning (0.00 sec)做查询如果数据量大的话 用 Lucene, Solr, ElasticSearch 不香吗?
字符串不加单引号索引失效(mysql会自己做隐式转换)
- Demo 同3
少用or, 用它来连接时会索引失效
Demo:
mysql> explain select * from staffs where name = 'John' or name = 'ronnie'; +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | idx_staffs_nameAgePos | NULL | NULL | NULL | 3 | 66.67 | Using where | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from staffs where name = 'John' or age = 23; +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | idx_staffs_nameAgePos | NULL | NULL | NULL | 3 | 55.56 | Using where | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from staffs where name = 'John' or pos = 'manager'; +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | staffs | NULL | ALL | idx_staffs_nameAgePos | NULL | NULL | NULL | 3 | 55.56 | Using where | +----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.01 sec)
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