由浅入深探究mysql索引结构原理、性能分析与优化（二）

(1.1)能正确的利用索引

l  Where子句表达式顺序是(username)

1. mysql> explain select * from one where username='abgvwfnt';  

2. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  

3. | id | select_type | table | type | possible_keys | key      | key_len | ref   |rows | Extra       |  

4. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  

5. |  1 | SIMPLE      | one   | ref  | username      | username | 24      | const |5 | Using where |  

6. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  

7. 1 row in set (0.00 sec)  

l  Where子句表达式顺序是(username,password)

1. mysql> explain select * from one where username='abgvwfnt' and password='123456';  

2. +----+-------------+-------+------+---------------+----------+---------+-------------+------+-------------+  

3. | id | select_type | table | type | possible_keys | key      | key_len | ref | rows | Extra       |  

4. +----+-------------+-------+------+---------------+----------+---------+-------------+------+-------------+  

5. |  1 | SIMPLE      | one   | ref  | username      | username | 43      | const,const |    1 | Using where |  

6. +----+-------------+-------+------+---------------+----------+---------+-------------+------+-------------+  

7. 1 row in set (0.00 sec)  

l  Where子句表达式顺序是(username,password, last_login)

1. mysql> explain select * from one where username='abgvwfnt' and password='123456'and last_login='1338251170';  

2. +----+-------------+-------+------+---------------+----------+---------+-------------------+------+-------------+  

3. | id | select_type | table | type | possible_keys | key      | key_len | ref| rows | Extra       |  

4. +----+-------------+-------+------+---------------+----------+---------+-------------------+------+-------------+  

5. |  1 | SIMPLE   | one   | ref  | username     | username | 83      | const,const,const |    1 | Using where |  

6. +----+-------------+-------+------+---------------+----------+---------+-------------------+------+-------------+  

7. 1 row in set (0.00 sec)  

上面可以看出type=ref 是多列索引，key_len分别是24、43、83，这说明用到的索引分别是(username), (username,password), (username,password, last_login );row分别是5、1、1检索的数据行都很少，因为这三个查询都按照索引前缀原则，可以利用到索引。

 

(1.2)不能正确的利用索引

l  Where子句表达式顺序是(password, last_login)

1. mysql> explain select * from one where password='123456'and last_login='1338251170';  

2. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

3. | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows| Extra       |  

4. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

5. |  1 | SIMPLE      | one   | ALL  | NULL          | NULL | NULL    | NULL | 20146 | Using where |  

6. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

7. 1 row in set (0.00 sec)  

l  Where 子句表达式顺序是(last_login)

1. mysql> explain select * from one where last_login='1338252525';  

2. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

3. | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows| Extra       |  

4. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

5. |  1 | SIMPLE      | one   | ALL  | NULL          | NULL | NULL    | NULL | 20146 | Using where |  

6. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

7. 1 row in set (0.00 sec)  

以上的两条语句都不是以username开始，这样是用不了索引，通过type=all(全表扫描)，key_len=null，rows都很大20146

Ps：one表里只有20003条数据，为什么出现20146，这是优化器对表的一个估算值，不精确的。

l  Where 子句表达式虽然顺序是(username,password, last_login)或(username,password)但第一个是有范围’<’、’>’，’<=’，’>=’等出现

1. mysql> explain select * from one where username>'abgvwfnt' and password ='123456'and last_login='1338251170';  

2. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

3. | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows| Extra       |  

4. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

5. |  1 | SIMPLE      | one   | ALL  | username      | NULL | NULL    | NULL | 20146 | Using where |  

6. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

7. 1 row in set (0.00 sec)  

这个查询很明显是遍历所有表，一个索引都没用到，非第一列出现范围(password列或last_login列)，则能利用索引到首先出现范围的一列，也就是“where username='abgvwfnt' and password >'123456'and last_login='1338251170';”或则“where username='abgvwfnt' and password >'123456'and last_login<'1338251170';”索引长度ref_len=43,索引检索到password列，所以考虑多列索引的时候把那些查询语句用的比较的列放在最后(或非第一位)。

l  断层，即是where顺序(username, last_login)

1. mysql> explain select * from one where username='abgvwfnt' and last_login='1338252525';  

2. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  

3. | id | select_type | table | type | possible_keys | key      | key_len | ref   | rows | Extra       |  

4. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  

5. |  1 | SIMPLE   | one   | ref  | username   | username | 24     | const |5 | Using where |  

6. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  

7. 1 row in set (0.00 sec)  

注意这里的key_len=24=8*3(8是username的长度，3是utf8编码)，rows=5，和下面一条sql语句搜索出来一样

1. mysql>  select * from one where username='abgvwfnt';  

2. +-------+----------+----------+-------+------------+  

3. | id    | username | password | level | last_login |  

4. +-------+----------+----------+-------+------------+  

5. |  3597 | abgvwfnt | 234567   |     0 | 1338251420 |  

6. |  7693 | abgvwfnt | 456789   |     0 | 1338251717 |  

7. | 11789 | abgvwfnt | 456789   |     0 | 1338251992 |  

8. | 15885 | abgvwfnt | 456789   |     0 | 1338252258 |  

9. | 19981 | abgvwfnt | 456789   |     0 | 1338252525 |  

10. +-------+----------+----------+-------+------------+  

11. 5 rows in set (0.00 sec)  

12.  

13. mysql>  select * from one where username='abgvwfnt' and last_login='1338252525';  

14. +-------+----------+----------+-------+------------+  

15. | id    | username | password | level | last_login |  

16. +-------+----------+----------+-------+------------+  

17. | 19981 | abgvwfnt | 456789   |     0 | 1338252525 |  

18. +-------+----------+----------+-------+------------+  

19. 1 row in set (0.00 sec)  

这个就是要的返回结果，所以可以知道断层(username,last_login)，这样只用到username索引，把用到索引的数据再重新检查last_login条件，这个相对全表查询来说还是有性能上优化，这也是很多sql优化文章中提到的where 范围查询要放在最后(这不绝对，但可以利用一部分索引)

(1.3)如果一个查询where子句中确实不需要password列，那就用“补洞”。

1. mysql> select distinct(password) from one;  

2. +----------+  

3. | password |  

4. +----------+  

5. | 234567   |  

6. | 345678   |  

7. | 456789   |  

8. | 123456   |  

9. +----------+  

10. 4 rows in set (0.08 sec)

可以看出password列中只有这几个值，当然在现实中不可能密码有这么多一样的，再说数据也可能不断更新，这里只是举例说明补洞的方法

1. mysql> explain select * from one where username='abgvwfnt' and password in('123456','234567','345678','456789')

2. and last_login='1338251170';  

3. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

4. | id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows | Extra       |  

5. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

6. |  1 | SIMPLE    | one | range | username    | username| 83      | NULL |4 | Using where |  

7. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

8. 1 row in set (0.00 sec)  

可以看出ref=83 所有的索引都用到了，type=range是因为用了in子句。

这个被“补洞”列中的值应该是有限的，可预知的，如性别，其值只有男和女(加多一个不男不女也无妨)。

“补洞”方法也有瓶颈，当很多列，且需要补洞的相应列(可以多列)的值虽有限但很多(如中国城市)的时候，优化器在优化时组合起来的数量是很大，这样的话就要做好基准测试和性能分析，权衡得失，取得一个合理的优化方法。

(1.4)like

1. mysql> explain select * from one where username like 'abgvwfnt%';  

2. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

3. | id | select_type | table | type  | possible_keys | key      | key_len | ref  |  

4. rows | Extra       |  

5. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

6. |  1 | SIMPLE      | one   | range | username      | username | 24      | NULL |  

7. 5 | Using where |  

8. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

9. 1 row in set (0.00 sec)  

10. mysql> explain select * from one where username like '%abgvwfnt%';  

11. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

12. | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows| Extra       |  

13. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

14. |  1 | SIMPLE      | one   | ALL  | NULL          | NULL | NULL    | NULL | 20259 | Using where |  

15. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

16. 1 row in set (0.01 sec)  

对比就知道like操作abgvwfnt%能用到索引，%abgvwfnt%用不到

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