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有些时候虽然有索引,但是不被优化器选择使用,下面是开发过程中遇到的不能使用索引的几种情况:
mysql> explain select * from actor where last_name like '%NI%';+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+| 1 | SIMPLE | actor | NULL | ALL | NULL | NULL | NULL | NULL | 200 | 11.11 | Using where |+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+1 row in set, 1 warning (0.42 sec)
数据库中InnoDB引擎默认使用B-Tree索引,再B-Tree索引结构中,以%开头的查询自然就法利用索引了。
一般都推荐使用全文索引(Full-text)来解决类似的全文检索问题。(这里没有给出全文索引的解决方案,可以自己查一下)
除此之外大家还可以利用InnoDB的表都是聚族表的特点,采取一种轻量级别的解决方式:
一般情况下,索引都会比表小,扫描索引比扫描表更快(特殊情况下,索引会比表更大)。
在InnoDB表上二级索引idx_last_name实际上存储字段last_name和主键actor_id,那么理想的访问方式应该是先扫描二级索引idx_last_name获得满足条件 last_name like '%NI%'的主键actor_id列表,之后根据主键回表去检索记录,这样就避免了全表扫描演员表actor产生的大量IO请求。
mysql> explain select * from (select actor_id from actor where last_name like '%NI%') a,actor b where a.actor_id = b.actor_id;+----+-------------+-------+------------+--------+---------------+---------------------+---------+-----------------------+------+----------+--------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+-------+------------+--------+---------------+---------------------+---------+-----------------------+------+----------+--------------------------+| 1 | SIMPLE | actor | NULL | index | PRIMARY | idx_actor_last_name | 137 | NULL | 200 | 11.11 | Using where; Using index || 1 | SIMPLE | b | NULL | eq_ref | PRIMARY | PRIMARY | 2 | sakila.actor.actor_id | 1 | 100.00 | NULL |+----+-------------+-------+------------+--------+---------------+---------------------+---------+-----------------------+------+----------+--------------------------+2 rows in set, 1 warning (0.00 sec)
通过执行计划中可以看到,内层查询的Using index 代表索引覆盖扫描,之后通过主键join操作去演员表actor中获取最终查询结果,理论上比直接扫描全表扫描更快一些。
特别是在当列类型是字符串时,那么一定记得在where条件中把字符常量的值用引号引起来,否则即便这个列上有索引,MYSQL也不会使用,因为MYSQL默认把常量值进行转换后才进行检索。
例如:演员表actor中姓氏字段last_name是字符型的,但是SQL语句的条件值1是一个数值类型,因此即便存在索引idx_last_name,MYSQL也不能正确使用索引,而是进行全表扫描。
mysql> explain select * from actor where last_name =1;+----+-------------+-------+------------+------+---------------------+------+---------+------+------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+-------+------------+------+---------------------+------+---------+------+------+----------+-------------+| 1 | SIMPLE | actor | NULL | ALL | idx_actor_last_name | NULL | NULL | NULL | 200 | 10.00 | Using where |+----+-------------+-------+------------+------+---------------------+------+---------+------+------+----------+-------------+1 row in set, 3 warnings (0.08 sec)
加上引号之后,再执行一次,就发先使用上索引了。
mysql> explain select * from actor where last_name ='1';+----+-------------+-------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+-------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+| 1 | SIMPLE | actor | NULL | ref | idx_actor_last_name | idx_actor_last_name | 137 | const | 1 | 100.00 | NULL |+----+-------------+-------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+1 row in set, 1 warning (0.10 sec)
最左匹配原则是要满足复合索引最左边的字段,如果查询条件不是第一个复合索引的第一个字段,则不符合最左匹配原则。
mysql> explain select * from payment where amount =3.98 and last_update ='2006-02-15 22:12:32';+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+-------------+| 1 | SIMPLE | payment | NULL | ALL | NULL | NULL | NULL | NULL | 16086 | 1.00 | Using where |+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+-------------+1 row in set, 1 warning (0.04 sec)
例如:查询以“S”开头的标题的电影,需要返回的记录比例较大,MySQL就预估索引扫描还不如全表扫描更快:
mysql> update film_text set title =concat('s',title);Query OK, 1000 rows affected (3.47 sec)Rows matched: 1000 Changed: 1000 Warnings: 0mysql> explain select * from film_text where title like 's%';+----+-------------+-----------+------------+------+-----------------------+------+---------+------+------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+-----------+------------+------+-----------------------+------+---------+------+------+----------+-------------+| 1 | SIMPLE | film_text | NULL | ALL | idx_title_description | NULL | NULL | NULL | 1000 | 11.11 | Using where |+----+-------------+-----------+------------+------+-----------------------+------+---------+------+------+----------+-------------+1 row in set, 1 warning (0.00 sec)
在MySQL 5.6版本中,能够通过Trace 清晰地看到优化器的选择过程,对用时少的进行选择。
mysql> explain select * from payment where customer_id =203 or amount =3.96;+----+-------------+---------+------------+------+--------------------+------+---------+------+-------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+---------+------------+------+--------------------+------+---------+------+-------+----------+-------------+| 1 | SIMPLE | payment | NULL | ALL | idx_fk_customer_id | NULL | NULL | NULL | 16086 | 10.15 | Using where |+----+-------------+---------+------------+------+--------------------+------+---------+------+-------+----------+-------------+1 row in set, 1 warning (0.00 sec)
因为or后面的条件列中没有索引,那么后面的查询肯定要走全表扫描,在存在全表扫描的情况下,就没有必要多做一次索引扫描增加I/O访问,一次全表扫过滤条件就足够了。
当or前后两个条件都有索引时,是会用到索引的。
mysql> explain select * from payment where customer_id= 203 or amount =3.96;+----+-------------+---------+------------+-------------+-------------------------------+-------------------------------+---------+------+------+----------+---------------------------------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+---------+------------+-------------+-------------------------------+-------------------------------+---------+------+------+----------+---------------------------------------------------------+| 1 | SIMPLE | payment | NULL | index_merge | idx_fk_customer_id,idx_amount | idx_fk_customer_id,idx_amount | 2,3 | NULL | 21 | 100.00 | Using union(idx_fk_customer_id,idx_amount); Using where |+----+-------------+---------+------------+-------------+-------------------------------+-------------------------------+---------+------+------+----------+---------------------------------------------------------+1 row in set, 1 warning (0.05 sec)
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