mysql 索引优化法则
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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