索引可以提高查询的速度，但并不是使用带有索引的字段查询时，索引都会起作用。使用索引有几种特殊情况，在这些情况下，有可能使用带有索引的字段查询时，索引并没有起作用，下面重点介绍这几种特殊情况。

1. 查询语句中使用LIKE关键字

在查询语句中使用 LIKE 关键字进行查询时，如果匹配字符串的第一个字符为“%”，索引不会被使用。如果“%”不是在第一个位置，索引就会被使用。

例 1

为了便于理解，我们先查询 tb_student 表中的数据，SQL 语句和运行结果如下：

mysql> select * from tb_student_info;

+----+--------+--------+------+------+--------+------------+

| id | name   | deptId | age  | sex  | height | login_date |

+----+--------+--------+------+------+--------+------------+

|  1 | Dany   |      1 |   25 | F    |    160 | 2015-09-10 |

|  2 | Green  |      3 |   23 | F    |    158 | 2016-10-22 |

|  3 | Henry  |      2 |   23 | M    |    185 | 2015-05-31 |

|  4 | Jane   |      1 |   22 | F    |    162 | 2016-12-20 |

|  5 | Jim    |      1 |   66 | M    |    175 | 2016-01-15 |

|  6 | John   |      2 |   21 | M    |    172 | 2015-11-11 |

|  7 | Lily   |      6 |   22 | F    |    165 | 2016-02-26 |

|  8 | Susan  |      4 |   23 | F    |    170 | 2015-10-01 |

|  9 | Thomas |      3 |   22 | M    |    178 | 2016-06-07 |

| 10 | Tom    |      4 |   23 | M    |    165 | 2016-08-05 |

| 12 | jijiji |      1 |  667 | 1    |    176 | 2020-08-24 |

+----+--------+--------+------+------+--------+------------+

11 rows in set (0.00 sec)

下面在查询语句中使用 LIKE 关键字，且匹配的字符串中含有“%”符号，使用 EXPLAIN 分析查询情况，SQL 语句和运行结果如下：

mysql> explain select * from tb_student_info where name like '%y';+----+-------------+-----------------+------------+------+---------------+------+---------+------+------+----------+-------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-----------------+------------+------+---------------+------+---------+------+------+----------+-------------+

| 1 | SIMPLE | tb_student_info | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 11.11 | Using where |

+----+-------------+-----------------+------------+------+---------------+------+---------+------+------+----------+-------------+

1 row in set, 1 warning (0.00sec)

mysql> alter table tb_student_info add indexindex_name (name);

Query OK,0 rows affected (0.51sec)

Records:0 Duplicates: 0 Warnings: 0mysql> explain select * from tb_student_info where name like '%y';+----+-------------+-----------------+------------+------+---------------+------+---------+------+------+----------+-------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-----------------+------------+------+---------------+------+---------+------+------+----------+-------------+

| 1 | SIMPLE | tb_student_info | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 11.11 | Using where |

+----+-------------+-----------------+------------+------+---------------+------+---------+------+------+----------+-------------+

1 row in set, 1 warning (0.00sec)

mysql> explain select * from tb_student_info where name like 'y%';+----+-------------+-----------------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-----------------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+

| 1 | SIMPLE | tb_student_info | NULL | range | index_name | index_name | 77 | NULL | 1 | 100.00 | Using index condition |

+----+-------------+-----------------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+

1 row in set, 1 warning (0.07 sec)

第一个查询语句执行后，rows 参数的值为 12，表示这次查询过程中查询了 12 条记录；创建索引之后第二个查询语句执行后，rows 参数的值为 12，表示这次查询过程只查询 12 条记录。第三次查询同样是使用 name 字段进行查询，因为第一个和第二个查询语句的 LIKE 关键字后的字符串是以“%”开头的，所以第二个查询语句没有使用索引(第一个查询的时候还没建立索引)，而第三个查询语句使用了索引 index_name。

注意: 关于explain的使用, 可以查看这里

2. 查询语句中使用多列索引

多列索引是在表的多个字段上创建一个索引，只有查询条件中使用了这些字段中的第一个字段，索引才会被使用。

例 2

在 name 和 age 两个字段上创建多列索引，并验证多列索引的使用情况，SQL 语句和运行结果如下：

mysql> alter table tb_student_info add indexindex_name_age (name, age);

Query OK,0 rows affected (0.56sec)

Records:0 Duplicates: 0 Warnings: 0

删除之前name列上的索引'index_name'

mysql> drop index index_name ontb_student_info;

Query OK,0 rows affected (0.25sec)

Records:0 Duplicates: 0 Warnings: 0

查看现在表tb_student_info上的索引

mysql> show index fromtb_student_info\G;*************************** 1. row ***************************

Table: tb_student_info

Non_unique:0Key_name:PRIMARYSeq_in_index:1Column_name: id

Collation: A

Cardinality:13Sub_part:NULLPacked:NULL

Null:

Index_type: BTREE

Comment:

Index_comment:

Visible: YES

Expression:NULL

*************************** 2. row ***************************

Table: tb_student_info

Non_unique:1Key_name: index_name_age

Seq_in_index:1Column_name: name

Collation: A

Cardinality:11Sub_part:NULLPacked:NULL

Null:

Index_type: BTREE

Comment:

Index_comment:

Visible: YES

Expression:NULL

*************************** 3. row ***************************

Table: tb_student_info

Non_unique:1Key_name: index_name_age

Seq_in_index:2Column_name: age

Collation: A

Cardinality:11Sub_part:NULLPacked:NULL

Null: YES

Index_type: BTREE

Comment:

Index_comment:

Visible: YES

Expression:NULL

3 rows in set (0.00 sec)

如上, 一个主键, 还有一个是组合索引, 组合字段是name和age...

执行分别查询name和age的sql, 如下:

mysql> explain select * from tb_student_info where name like 'y%';+----+-------------+-----------------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-----------------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+

| 1 | SIMPLE | tb_student_info | NULL | range | index_name_age | index_name_age | 77 | NULL | 1 | 100.00 | Using index condition |

+----+-------------+-----------------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+

1 row in set, 1 warning (0.00sec)

mysql> explain select * from tb_student_info where age = 12;+----+-------------+-----------------+------------+------+---------------+------+---------+------+------+----------+-------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-----------------+------------+------+---------------+------+---------+------+------+----------+-------------+

| 1 | SIMPLE | tb_student_info | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 10.00 | Using where |

+----+-------------+-----------------+------------+------+---------------+------+---------+------+------+----------+-------------+

1 row in set, 1 warning (0.00 sec)

第一条查询语句的查询条件使用了 name 字段，分析结果显示 rows 参数的值为 1，且查询过程中使用了 index_name_age 索引。第二条查询语句的查询条件使用了 age 字段，结果显示 rows 参数的值为 12，且 key 参数的值为 NULL，这说明第二个查询语句没有使用索引。

因为 name 字段是多列索引的第一个字段，所以只有查询条件中使用了 name 字段才会使 index_name_age 索引起作用。

mysql> explain select * from tb_student_info where age=23 and name = 'Green';+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------------+------+----------+-------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------------+------+----------+-------+

| 1 | SIMPLE | tb_student_info | NULL | ref | index_name_age | index_name_age | 82 | const,const | 1 | 100.00 | NULL |

+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------------+------+----------+-------+

1 row in set, 1 warning (0.00sec)

mysql> explain select * from tb_student_info where name = 'Green' and age = 23;+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------------+------+----------+-------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------------+------+----------+-------+

| 1 | SIMPLE | tb_student_info | NULL | ref | index_name_age | index_name_age | 82 | const,const | 1 | 100.00 | NULL |

+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------------+------+----------+-------+

1 row in set, 1 warning (0.00 sec)

如上, 查询中的where条件中用到了name和age两列, 可以看出, 不管where条件中先写age还是name都对使用索引index_name_age没有影响, 因为两个sql中的where条件中都有name列, 根据索引index_name_age的第一个字段就是name, 所以索引被使用

mysql> explain select * from tb_student_info where name = 'Green' and sex = 'F';+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------+------+----------+-------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------+------+----------+-------------+

| 1 | SIMPLE | tb_student_info | NULL | ref | index_name_age | index_name_age | 77 | const | 1 | 10.00 | Using where |

+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------+------+----------+-------------+

1 row in set, 1 warning (0.00 sec)

where条件中使用了name和sex, 依然使用了索引, 因为where条件中的name得以使用索引...

3. 查询语句中使用OR关键字

查询语句只有 OR 关键字时，如果 OR 前后的两个条件的列都是索引，那么查询中将使用索引。如果 OR 前后有一个条件的列不是索引，那么查询中将不使用索引。

例 3

下面演示 OR 关键字的使用。

mysql> explain select * from tb_student_info where name='Green' or age>1;+----+-------------+-----------------+------------+------+----------------+------+---------+------+------+----------+-------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-----------------+------------+------+----------------+------+---------+------+------+----------+-------------+

| 1 | SIMPLE | tb_student_info | NULL | ALL | index_name_age | NULL | NULL | NULL | 12 | 40.00 | Using where |

+----+-------------+-----------------+------------+------+----------------+------+---------+------+------+----------+-------------+

1 row in set, 1 warning (0.00sec)

mysql> explain select * from tb_student_info where name='Green' or (name='Green' and age=23);+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------+------+----------+-------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------+------+----------+-------+

| 1 | SIMPLE | tb_student_info | NULL | ref | index_name_age | index_name_age | 77 | const | 1 | 100.00 | NULL |

+----+-------------+-----------------+------------+------+----------------+----------------+---------+-------+------+----------+-------+

1 row in set, 1 warning (0.00sec)

mysql> explain select * from tb_student_info where name='Green' or id=1;+----+-------------+-----------------+------------+-------------+------------------------+------------------------+---------+------+------+----------+-------------------------------------------------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered |Extra|

+----+-------------+-----------------+------------+-------------+------------------------+------------------------+---------+------+------+----------+-------------------------------------------------------+

| 1 | SIMPLE | tb_student_info | NULL | index_merge | PRIMARY,index_name_age | index_name_age,PRIMARY | 77,4 | NULL | 2 | 100.00 | Using sort_union(index_name_age,PRIMARY); Using where |

+----+-------------+-----------------+------------+-------------+------------------------+------------------------+---------+------+------+----------+-------------------------------------------------------+

1 row in set, 1 warning (0.00 sec)

由于 单独的age字段没有索引，所以第一条查询语句没有使用索引, 因为or后面的age单独使用, 没有涉及到索引index_name_age第一个字段name的使用；第二个查询or前面的name和or后面的都设计到了索引index_name_age, 所以使用了此索引; 而最后一个查询name 字段和 id 字段都有索引, 使用了 index_name_age 和 PRIMARY 索引

其实综上一定要记住, 组合索引的使用, 一定要遵循最左匹配原则!

总结

使用索引查询记录时，一定要注意索引的使用情况。例如，LIKE 关键字配置的字符串不能以“%”开头；使用多列索引时，查询条件必须要使用这个索引的第一个字段；使用 OR 关键字时，OR 关键字连接的所有条件都必须使用索引。