时间：2021-07-01 10:21:17 帮助过：22人阅读
CREATE TABLE staffs( id INT(10) PRIMARY KEY AUTO_INCREMENT, NAME VARCHAR(24) NOT NULL DEFAULT ‘‘ COMMENT ‘姓名‘, age INT(10) NOT NULL DEFAULT 0 COMMENT ‘年龄‘, pos VARCHAR(24) NOT NULL DEFAULT ‘‘ 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(‘z3‘,22,‘manager‘,NOW()); INSERT INTO staffs(NAME,age,pos,add_time) VALUES(‘July‘,23,‘dev‘,NOW()); INSERT INTO staffs(NAME,age,pos,add_time) VALUES(‘2000‘,23,‘dev‘,NOW()); ALTER TABLE staffs ADD INDEX idx_staffs_nameAgePos(NAME,age,pos);
mysql> show index from staffs; +--------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +--------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | staffs | 0 | PRIMARY | 1 | id | A | 3 | NULL | NULL | | BTREE | | | | staffs | 1 | idx_staffs_nameAgePos | 1 | NAME | A | 3 | NULL | NULL | | BTREE | | | | staffs | 1 | idx_staffs_nameAgePos | 2 | age | A | 3 | NULL | NULL | | BTREE | | | | staffs | 1 | idx_staffs_nameAgePos | 3 | pos | A | 3 | NULL | NULL | | BTREE | | | +--------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 4 rows in set (0.00 sec)
规则1：全值匹配我最爱
mysql> explain select * from staffs where name=‘July‘; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | Using index condition | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ 1 row in set (0.01 sec) mysql> explain select * from staffs where name=‘July‘ and age=23; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 78 | const,const | 1 | Using index condition | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+ 1 row in set (0.00 sec) mysql> explain select * from staffs where name=‘July‘ and age=23 and pos=‘dev‘; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 152 | const,const,const | 1 | Using index condition | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+ 1 row in set (0.00 sec)
第三种效果最好~ 用到了我们创建的索引，下面再看一种情况
mysql> explain select * from staffs where age=23 and pos=‘dev‘; +----+-------------+--------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | staffs | ALL | NULL | NULL | NULL | NULL | 3 | Using where | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.01 sec) mysql> explain select * from staffs where pos=‘dev‘; +----+-------------+--------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | staffs | ALL | NULL | NULL | NULL | NULL | 3 | Using where | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)
全表扫描！！我们看到，索引竟然失效了
mysql> explain select * from staffs where name=‘July‘; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | Using index condition | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ 1 row in set (0.00 sec)
单独使用name就不会使索引失效，由此引入规则2
规则2：最佳左前缀法则，即如果索引了多列，要遵守最佳左前缀法则，即查询必须从索引的最左前列开始，并且不跳过索引中的列~
ALTER TABLE staffs ADD INDEX idx_staffs_nameAgePos(name,age,pos);
索引的最左前列是name,name相当于火车头，没有火车头，搞毛~所以索引的第一个字段不能丢失
再来看一种情况
mysql> explain select * from staffs where name=‘July‘ and pos=‘dev‘; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | Using index condition | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ 1 row in set (0.00 sec)
只有一个const,这和只用name是一样的，违背了什么原则呢？不跳过索引中的列
mysql> explain select * from staffs where name=‘July‘ and age=23; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 78 | const,const | 1 | Using index condition | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+ 1 row in set (0.00 sec)
以上两个对比一下即可~
规则3.不要在索引列上做任何操作(计算，函数，类型转换)，会导致索引失效而转向全表扫描~
mysql> select * from staffs where name=‘July‘; +----+------+-----+-----+---------------------+ | id | NAME | age | pos | add_time | +----+------+-----+-----+---------------------+ | 2 | July | 23 | dev | 2017-03-17 23:14:23 | +----+------+-----+-----+---------------------+ 1 row in set (0.00 sec) mysql> select * from staffs where left(name,4)=‘July‘; +----+------+-----+-----+---------------------+ | id | NAME | age | pos | add_time | +----+------+-----+-----+---------------------+ | 2 | July | 23 | dev | 2017-03-17 23:14:23 | +----+------+-----+-----+---------------------+ 1 row in set (0.04 sec) mysql> explain select * from staffs where name=‘July‘; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | Using index condition | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ 1 row in set (0.00 sec) mysql> explain select * from staffs where left(name,4)=‘July‘; +----+-------------+--------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | staffs | ALL | NULL | NULL | NULL | NULL | 3 | Using where | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)
规则4.存储引擎不能使用索引中范围条件右边的列
mysql> explain select * from staffs where name=‘July‘ and age=23 and pos=‘dev‘; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 152 | const,const,const | 1 | Using index condition | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+ 1 row in set (0.00 sec) mysql> explain select * from staffs where name=‘July‘ and age>23 and pos=‘dev‘; +----+-------------+--------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+ | 1 | SIMPLE | staffs | range | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 78 | NULL | 1 | Using index condition | +----+-------------+--------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+ 1 row in set (0.00 sec)
age>23导致后面的pos索引失效
规则5.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 *
mysql> explain select * from staffs where name=‘July‘ and age=23 and pos=‘dev‘; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 152 | const,const,const | 1 | Using index condition | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+ 1 row in set (0.00 sec) mysql> explain select name,age,pos from staffs where name=‘July‘ and age=23 and pos=‘dev‘; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+--------------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 152 | const,const,const | 1 | Using where; Using index | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------------------+------+--------------------------+ 1 row in set (0.00 sec) mysql> explain select name,age,pos from staffs where name=‘July‘ and age>23 and pos=‘dev‘; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+--------------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | Using where; Using index | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+--------------------------+ 1 row in set (0.00 sec) mysql> explain select * from staffs where name=‘July‘ and age>23 and pos=‘dev‘; +----+-------------+--------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+ | 1 | SIMPLE | staffs | range | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 78 | NULL | 1 | Using index condition | +----+-------------+--------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+ 1 row in set (0.00 sec)
规则6.MySQL中在使用不等于(!=或者<>)的时候无法使用索引导致全表扫描
mysql> explain select * from staffs where name=‘July‘; +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | Using index condition | +----+-------------+--------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+ 1 row in set (