MySQL 通过多个示例学习索引
Posted 寻觅beyond
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最近在准备面试，关于索引这一块，发现很多以前忽略的点，这里好好整理一下
首先为什么要建立索引
一本书，有章、节、段、行这种单位。
如果现在需要找一个内容：第9章>第2节>第3段>第4行>第5个字。
如果没有索引（目录），那么，就必须从第一页开始，按照这个顺序：第1章>第1节>第1段>第1行>第1个字开始，顺序查找，也就是说，如果运气足够坏，那么，找到目标的时候，已经将整本书都差不多扫了一遍。
但是现在，如果建立了索引（目录），只是建立了 “章” 的索引：每一章的第一页，那么，我们开始查找的时候，并不是从第1章开始，我们可以根据目录，查找到第9章的页数，然后直接翻到第9章的那章内容，初始就是第9章>第1节>第1段>第1行>第1个字，然后仍旧开始顺序查找，那么这个时候，虽然仍然有点慢，但是已经快了很多很多。
现在，如果建立（章-节）的索引，记录每一章的每一个小节的第1页，那么我们开始查找的时候，可以通过读一下索引，直接找到第9章的第2小节，然后从这1小节的第1段的第1行的第1个字开始查找。仍旧顺序查找。
很好，你会想，可以建立（章-节-段）的索引，那么，一开始就可以直接定位到第9章>第2小节>第3段，然后从这段的第1行的第1个字开始顺序查找。
那么，有没有必要建立（章-节-段-行）的索引呢？可以根据情况而定。
索引类型
有很多中索引，比如：
1、普通索引：最基本的索引，没有任何约束限制
2、唯一索引：与主键索引类似，但是不允许出现相同的值（唯一性约束）。
3、主键索引：特殊的唯一索引，不允许有空值。
4、复盖索引（组合索引）：将多个列组合在一起创建索引，可以覆盖多个列，比如“章-节-段”建立索引
5、外键索引：只有Innodb支持，保证数据一致性、完整性，实现级联操作。
6、全文索引：只有MyISAM引擎支持，并且只支持对英文进行全文检索。
建立索引
假设有一个user表（用户表）
mysql> desc user; +-------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+----------------+ | uid | int(11) | NO | PRI | NULL | auto_increment | | uname | char(20) | NO | UNI | NULL | | | addr | char(20) | NO | MUL | NULL | | | token | char(20) | NO | | NULL | | +-------+----------+------+-----+---------+----------------+
uid因为是主键，所以已经默认创建了主键索引
为uname字段建立一个唯一索引，为addr字段建立普通索引：
token字段不建立索引
mysql> create unique index uname_index on user(uname); mysql> create index addr_index on user(addr);
查看索引
mysql> show index from user;
*************************** 1. row ***************************
Table: user
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 1
Column_name: uid
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
*************************** 2. row ***************************
Table: user
Non_unique: 0
Key_name: uname_index
Seq_in_index: 1
Column_name: uname
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
*************************** 3. row ***************************
Table: user
Non_unique: 1
Key_name: addr_index
Seq_in_index: 1
Column_name: addr
Collation: A
Cardinality: NULL
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
3 rows in set (0.00 sec)
插入数据
使用pdo循环插入10000条记录，各个字段除了uid自增之外，都是随机值。
<?php $pdo = new PDO("mysql:host=127.0.0.1;dbname=MyDb;charset=utf8", "root", "root"); $stmt = $pdo->prepare("insert into user (uname, addr, token) values (?, ?, ?)"); $str = "abcdefghijklmnopqrstuvwxzy"; for ($j = 0; $j < 10000; $j++) { $name = ""; $addr = ""; $token = ""; for ($i = 0; $i < 7; $i++) { $name .= $str[mt_rand(0, 25)]; $addr .= $str[mt_rand(0, 25)]; $token .= $str[mt_rand(0, 25)]; } $stmt->execute(array($name, $addr, $token)); } ?>
是否会使用索引？
示例1：
mysql> explain select count(*) from user; +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away | +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+ 1 row in set (0.00 sec)
未使用索引，因为没有使用where子句，索引是在查找的时候（有where的时候才使用索引）。
示例2：
mysql> explain select * from user where uname=‘outalwa‘; +----+-------------+-------+-------+---------------+-------------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+-------------+---------+-------+------+-------+ | 1 | SIMPLE | user | const | uname_index | uname_index | 60 | const | 1 | | +----+-------------+-------+-------+---------------+-------------+---------+-------+------+-------+ 1 row in set (0.00 sec) mysql> explain select * from user where uname!=‘outalwa‘; +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | 1 | SIMPLE | user | ALL | uname_index | NULL | NULL | NULL | 10000 | Using where | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ 1 row in set (0.00 sec) mysql> explain select * from user where uname<>‘outalwa‘; +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | 1 | SIMPLE | user | ALL | uname_index | NULL | NULL | NULL | 10000 | Using where | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ 1 row in set (0.00 sec)
对于一个建了索引的列来说，如果使用=、!=、<>来判断字段值是否与一个值相等的时候，只有使用=的时候会使用索引，而!=和<>不会使用索引。
示例3：
mysql> explain select * from user where uid=‘9999‘ and uname=‘muqehaq‘; +----+-------------+-------+-------+---------------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | user | const | PRIMARY,uname_index | PRIMARY | 4 | const | 1 | | +----+-------------+-------+-------+---------------------+---------+---------+-------+------+-------+ 1 row in set (0.06 sec)
因为uid建立了主键索引，uname建立了唯一索引，使用and表示（并且）的关系时，会使用各自的索引。只扫描了1行数据就找到了
示例4：
mysql> explain select * from user where uid=‘9999‘ or uname=‘muqehaq‘; +----+-------------+-------+-------------+---------------------+---------------------+---------+------+------+-----------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------------+---------------------+---------------------+---------+------+------+-----------------------------------------------+ | 1 | SIMPLE | user | index_merge | PRIMARY,uname_index | PRIMARY,uname_index | 4,60 | NULL | 2 | Using union(PRIMARY,uname_index); Using where | +----+-------------+-------+-------------+---------------------+---------------------+---------+------+------+-----------------------------------------------+ 1 row in set (0.00 sec)
因为uid建立了主键索引，uname建立了唯一索引，使用or表示（或）的关系时，使用了索引。
示例5
mysql> explain select * from user where uid=‘9999‘ and token=‘dakghzz‘; +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | user | const | PRIMARY | PRIMARY | 4 | const | 1 | | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
uid有主键索引，token没有建立索引，使用and来连接条件，可以使用索引。
如果条件是token=‘dakghzz‘ and uid=9999 ，同样会使用索引。
示例6
mysql> explain select * from user where uid=‘9999‘ or token=‘dakghzz‘; +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | 1 | SIMPLE | user | ALL | PRIMARY | NULL | NULL | NULL | 10000 | Using where | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ 1 row in set (0.00 sec)
uid有主键索引，但是token没有建立索引，使用or来连接的时候，可以看到，查询并没有使用索引。
示例7
mysql> explain select count(*) from user where uid < 1000; +----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+ | 1 | SIMPLE | user | range | PRIMARY | PRIMARY | 4 | NULL | 1401 | Using where; Using index | +----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+ 1 row in set (0.00 sec) mysql> explain select count(*) from user where uid between 10 and 1000; +----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+ | 1 | SIMPLE | user | range | PRIMARY | PRIMARY | 4 | NULL | 1388 | Using where; Using index | +----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+ 1 row in set (0.00 sec) mysql> explain select count(*) from user where uid >10 and uid < 1000; +----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+ | 1 | SIMPLE | user | range | PRIMARY | PRIMARY | 4 | NULL | 1386 | Using where; Using index | +----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+ 1 row in set (0.00 sec)
可以看到，对一个创建了索引的列，使用 大于、小于、大于等于、小于等于、between and 最为筛选条件，都会使用索引。
示例8
mysql> explain select * from user where uname=null; +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables | +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ 1 row in set (0.00 sec) mysql> explain select * from user where uname!=null; +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables | +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ 1 row in set (0.00 sec) mysql> explain select * from user where uname is null; +----+-------------+-------+------+---------------+------+---------+------+------+------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE | +----+-------------+-------+------+---------------+------+---------+------+------+------------------+ 1 row in set (0.00 sec) mysql> explain select * from user where uname is not null; +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | 1 | SIMPLE | user | ALL | uname_index | NULL | NULL | NULL | 10000 | Using where | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ 1 row in set (0.00 sec)
对于一个建立了索引的列来说，判断字段是否为0时，使用=，!=，is null这几种方式都不会使用索引，但是is not null可以使用索引。
示例9
mysql> explain select * from user where uname like ‘%wolwgrc‘; +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | 1 | SIMPLE | user | ALL | NULL | NULL | NULL | NULL | 10000 | Using where | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ 1 row in set (0.00 sec) mysql> explain select * from user where uname like ‘%wolwgrc%‘; +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | 1 | SIMPLE | user | ALL | NULL | NULL | NULL | NULL | 10000 | Using where | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ mysql> explain select * from user where uname like ‘wolwgrc‘; +----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+ | 1 | SIMPLE | user | range | uname_index | uname_index | 60 | NULL | 1 | Using where | +----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+ 1 row in set (0.00 sec) mysql> explain select * from user where uname like ‘wolwgrc%‘; +----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+ | 1 | SIMPLE | user | range | uname_index | uname_index | 60 | NULL | 1 | Using where | +----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+ 1 row in set (0.00 sec)
对一个已经建立了索引的列使用模式匹配的时候，%keyword、%keyword%不会使用索引，而keyword和keyword%会使用索引。
普通索引的注意事项
1、 对于一个没有where的查询，不会使用索引，所以，在创建索引的时候，应该以where子句的字段为准，而不是select后面的字段为准。
2、对于一个建了索引的列来说，如果使用=、!=、<>来判断字段值是否与一个值相等的时候，只有使用=的时候会使用索引，而!=和<>不会使用索引。
3、where包含多个字段，如果每个字段都建了索引，那么使用and和or连接的时候，查询会使用索引。
4、where包含多个字段，如果一部分字段建立了索引，而有一部分字段没有建立索引，那么使用and连接的时候，查询的时候，建立了索引的字段会使用索引，而没有建立索引的字段不会使用索引。
5、where包含多个字段，如果一部分字段建立了索引，而有一部分字段没有建立索引，那么使用or连接的时候，查询的时候，不会使用索引。
6、对一个创建了索引的列，使用 大于、小于、大于等于、小于等于、between and 最为筛选条件，都会使用索引。
7、对于一个建立了索引的列来说，判断字段是否为0时，使用=，!=，is null这几种方式都不会使用索引，但是is not null可以使用索引。
8、对一个已经建立了索引的列使用模式匹配的时候，%keyword、%keyword%不会使用索引，而keyword和keyword%会使用索引。
复合索引
复合索引有前缀原则，后面示例会解释。
删除之前为uname字段和addr字段创建的索引。
创建一个复合索引，包含uname、addr、token，命令如下：
mysql> drop index uname_index on user;
Query OK, 10000 rows affected (0.27 sec)
Records: 10000 Duplicates: 0 Warnings: 0
mysql> drop index addr_index on user;
Query OK, 10000 rows affected (0.24 sec)
Records: 10000 Duplicates: 0 Warnings: 0
mysql> create index three_fields_index on user(uname, addr, token);
Query OK, 10000 rows affected (0.25 sec)
Records: 10000 Duplicates: 0 Warnings: 0
mysql> show index from user;
+-------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| user | 0 | PRIMARY | 1 | uid | A | 10000 | NULL | NULL | | BTREE | | |
| user | 1 | three_fields_index | 1 | uname | A | 10000 | NULL | NULL | | BTREE | | |
| user | 1 | three_fields_index | 2 | addr | A | 10000 | NULL | NULL | | BTREE | | |
| user | 1 | three_fields_index | 3 | token | A | 10000 | NULL | NULL | | BTREE | | |
+-------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
可以从上面索引查询结果中看出，uname、addr、token都包含在一个索引中（Key_name相同）。
覆盖索引的使用示例
下面的实例，简化一下，创建的索引是key(uname, addr, token)，那么对应key(A, B, C)
示例1
mysql> explain select * from user where uname=‘muqehaq‘ and addr=‘hgommlp‘ and token=‘dakghzz‘; +----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+ | 1 | SIMPLE | user | ref | three_fields_index | three_fields_index | 180 | const,const,const | 1 | Using where | +----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+ 1 row in set (0.00 sec) mysql> explain select * from user where addr=‘hgommlp‘ and token=‘dakghzz‘ and uname=‘muqehaq‘; +----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+ | 1 | SIMPLE | user | ref | three_fields_index | three_fields_index | 180 | const,const,const | 1 | Using where | +----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+ 1 row in set (0.00 sec)
因为建立了uname-addr-token的联合索引，在where中，三个字段都出现了（和建立索引时使用的字段列相同时），此时，和每个字段的顺序无关，都会使用索引。
结论：where A=‘xx‘ and B=‘xx‘ and 或者where C=‘xx‘ and B=‘xx‘ and A=‘xx‘会使用索引。
示例2
mysql> explain select * from user where uname=‘muqehaq‘ and addr=‘hgommlp‘; +----+-------------+-------+------+--------------------+--------------------+---------+-------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+--------------------+--------------------+---------+-------------+------+-------------+ | 1 | SIMPLE | user | ref | three_fields_index | three_fields_index | 120 | const,const | 1 | Using where | +----+-------------+-------+------+--------------------+--------------------+---------+-------------+------+-------------+ 1 row in set (0.00 sec)
示例：where A=‘xx‘ and B=‘xx‘
结论：会使用索引，使用复合索引中的前两个字段的索引。
示例3：
mysql> explain select * from user where uname=‘muqehaq‘ and token=‘dakghzz‘; +----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+ | 1 | SIMPLE | user | ref | three_fields_index | three_fields_index | 60 | const | 1 | Using where | +----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+ 1 row in set (0.00 sec) mysql> explain select * from user where token=‘dakghzz‘ and uname=‘muqehaq‘; +----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+ | 1 | SIMPLE | user | ref | three_fields_index | three_fields_index | 60 | const | 1 | Using where | +----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+ 1 row in set (0.00 sec)
示例：where A=‘xxx‘ and C=‘xx‘ 或者 where C=‘xx‘ and A=‘xxx‘
结论：查询A字段的时候，会使用复合索引中的A那一部分，但是查询C字段的时候，并不会使用复合索引。
原因：因为没有使用复合索引中的B字段。
示例4
mysql> explain select * from user where addr=‘hgommlp‘ and token=‘dakghzz‘; +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | 1 | SIMPLE | user | ALL | NULL | NULL | NULL | NULL | 10000 | Using where | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ 1 row in set (0.00 sec)
示例：where B=‘xxx‘ and C=‘xxx‘
结论：不会使用索引。
原因：没有使用复合索引中B前面的A字段索引。
示例5
mysql> explain select * from user where uname=‘muqehaq‘;
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+
| 1 | SIMPLE | user | ref | three_fields_index | three_fields_index | 60 | const | 1 | Using where |
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select * from user where addr=‘hgommlp‘;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| 1 | SIMPLE | user | ALL | NULL | NULL | NULL | NULL | 10000 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)
mysql> explain select * from user where token=‘dakghzz‘;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| 1 | SIMPLE | user | ALL | NULL | NULL | NULL | NULL | 10000 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
示例：where A=‘xxx‘ 结论：会使用索引。
示例：where B=‘xxx‘ 结论：不会使用索引。
示例：where C=‘xxx‘ 结论：不会使用索引。
复合索引的使用总结
假设创建了key（a，b，c）复合索引，那么：
where a = 1 and b = 2 and c = 3; 会使用索引
where a = 1 and c = 3 and b = 2; 会使用索引
where a = 1 and b = 2 ； 会使用索引
where a = 1;会使用索引
where a = 1 and c = 3; 不会使用索引，因为c前面的b没有使用
where b = 2 and c = 3; 不会使用索引，因为b前面的a没有使用
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