索引的最左前缀原则

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联合索引 / 复合索引

为什么会有最左前缀原则

常言，知其然更要知其所以然。
为什么会有最左前缀原则呢？
这其实是跟索引的存储结构相关的，详情可以查看索引背后的数据结构与算法。

什么是最左前缀原则

MySQL中，索引可以以一定的顺序引用多个列，这种索引就叫做联合索引（或复合索引）。
一般的，一个联合索引是一个有序元组，其中各个元素均为数据表的一列。

如有表titles如下：

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SHOW INDEX FROM titles;

+--------+------------+----------+--------------+-------------+-----------+-------------+------+------------+
| Table  | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Null | Index_type |
+--------+------------+----------+--------------+-------------+-----------+-------------+------+------------+
| titles |          0 | PRIMARY  |            1 | emp_no      | A         |        NULL |      | BTREE      |
| titles |          0 | PRIMARY  |            2 | title       | A         |        NULL |      | BTREE      |
| titles |          0 | PRIMARY  |            3 | from_date   | A         |      443308 |      | BTREE      |
+--------+------------+----------+--------------+-------------+-----------+-------------+------+------------+

情况一：全列匹配

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EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001' AND title='Senior Engineer' AND from_date='1986-06-26';

+----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref               | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+
|  1 | SIMPLE      | titles | const | PRIMARY       | PRIMARY | 59      | const,const,const |    1 |       |
+----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+

很明显，当按照索引中所有列进行精确匹配（这里精确匹配指“=”或“IN”匹配）时，索引可以被用到。

这里有一点需要注意，理论上索引对顺序是敏感的，但是由于MySQL的查询优化器会自动调整where子句的条件顺序以使用适合的索引，例如我们将where中的条件顺序颠倒：

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EXPLAIN SELECT * FROM employees.titles WHERE from_date='1986-06-26' AND emp_no='10001' AND title='Senior Engineer';

+----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref               | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+
|  1 | SIMPLE      | titles | const | PRIMARY       | PRIMARY | 59      | const,const,const |    1 |       |
+----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+

效果是一样的。

情况二：最左前缀匹配

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EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001';

+----+-------------+--------+------+---------------+---------+---------+-------+------+-------+
| id | select_type | table  | type | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+--------+------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | titles | ref  | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+--------+------+---------------+---------+---------+-------+------+-------+

当查询条件精确匹配索引的左边连续一个或几个列时，如或，所以可以被用到，但是只能用到一部分，即条件所组成的最左前缀。上面的查询从分析结果看用到了PRIMARY索引，但是key_len为4，说明只用到了索引的第一列前缀。

情况三：查询条件用到了索引中列的精确匹配，但是中间某个条件未提供

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EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001' AND from_date='1986-06-26';

+----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table  | type | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+
|  1 | SIMPLE      | titles | ref  | PRIMARY       | PRIMARY | 4       | const |    1 | Using where |
+----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+

此时索引使用情况和情况二相同，因为title未提供，所以查询只用到了索引的第一列，而后面的from_date虽然也在索引中，但是由于title不存在而无法和左前缀连接，因此需要对结果进行扫描过滤from_date（这里由于emp_no唯一，所以不存在扫描）。如果想让from_date也使用索引而不是where过滤，可以增加一个辅助索引，此时上面的查询会使用这个索引。除此之外，还可以使用一种称之为“隔离列”的优化方法，将emp_no与from_date之间的“坑”填上。

首先我们看下title一共有几种不同的值：

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SELECT DISTINCT(title) FROM employees.titles;
+--------------------+
| title              |
+--------------------+
| Senior Engineer    |
| Staff              |
| Engineer           |
| Senior Staff       |
| Assistant Engineer |
| Technique Leader   |
| Manager            |
+--------------------+

只有7种。在这种成为“坑”的列值比较少的情况下，可以考虑用“IN”来填补这个“坑”从而形成最左前缀：

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EXPLAIN SELECT * FROM employees.titles
WHERE emp_no='10001'
AND title IN ('Senior Engineer', 'Staff', 'Engineer', 'Senior Staff', 'Assistant Engineer', 'Technique Leader', 'Manager')
AND from_date='1986-06-26';

+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | titles | range | PRIMARY       | PRIMARY | 59      | NULL |    7 | Using where |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+

这次key_len为59，说明索引被用全了，但是从type和rows看出IN实际上执行了一个range查询，这里检查了7个key。看下两种查询的性能比较：

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SHOW PROFILES;

+----------+------------+-------------------------------------------------------------------------------+
| Query_ID | Duration   | Query                                                                         |
+----------+------------+-------------------------------------------------------------------------------+
|       10 | 0.00058000 | SELECT * FROM employees.titles WHERE emp_no='10001' AND from_date='1986-06-26'|
|       11 | 0.00052500 | SELECT * FROM employees.titles WHERE emp_no='10001' AND title IN ...          |
+----------+------------+-------------------------------------------------------------------------------+

“填坑”后性能提升了一点。如果经过emp_no筛选后余下很多数据，则后者性能优势会更加明显。当然，如果title的值很多，用填坑就不合适了，必须建立辅助索引。

情况四：查询条件没有指定索引第一列

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EXPLAIN SELECT * FROM employees.titles WHERE from_date='1986-06-26';

+----+-------------+--------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | titles | ALL  | NULL          | NULL | NULL    | NULL | 443308 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+--------+-------------+

由于不是最左前缀，索引这样的查询显然用不到索引。

情况五：匹配某列的前缀字符串

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EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001' AND title LIKE 'Senior%';

+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | titles | range | PRIMARY       | PRIMARY | 56      | NULL |    1 | Using where |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+

此时可以用到索引，但是如果通配符%出现在开头，则无法使用索引。

情况六：范围查询

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EXPLAIN SELECT * FROM employees.titles WHERE emp_no < '10010' and title='Senior Engineer';

+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | titles | range | PRIMARY       | PRIMARY | 4       | NULL |   16 | Using where |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+

范围列可以用到索引（必须是最左前缀），但是范围列后面的列无法用到索引。同时，索引最多用于一个范围列，因此如果查询条件中有两个范围列则无法全用到索引。

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EXPLAIN SELECT * FROM employees.titles
WHERE emp_no < '10010'
AND title='Senior Engineer'
AND from_date BETWEEN '1986-01-01' AND '1986-12-31';

+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | titles | range | PRIMARY       | PRIMARY | 4       | NULL |   16 | Using where |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+

可以看到索引对第二个范围索引无能为力。

这里特别要说明MySQL一个有意思的地方，那就是仅用explain可能无法区分范围索引和多值匹配，因为在type中这两者都显示为range。同时，用了“between”并不意味着就是范围查询，例如下面的查询：

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EXPLAIN SELECT * FROM employees.titles
WHERE emp_no BETWEEN '10001' AND '10010'
AND title='Senior Engineer'
AND from_date BETWEEN '1986-01-01' AND '1986-12-31';

+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | titles | range | PRIMARY       | PRIMARY | 59      | NULL |   16 | Using where |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+

看起来是用了两个范围查询，但作用于emp_no上的“BETWEEN”实际上相当于“IN”，也就是说emp_no实际是多值精确匹配。可以看到这个查询用到了索引全部三个列。因此在MySQL中要谨慎地区分多值匹配和范围匹配，否则会对MySQL的行为产生困惑。

情况七：查询条件中含有函数或表达式

很不幸，如果查询条件中含有函数或表达式，则MySQL不会为这列使用索引（虽然某些在数学意义上可以使用）。例如：

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EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001' AND left(title, 6)='Senior';
+----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table  | type | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+
|  1 | SIMPLE      | titles | ref  | PRIMARY       | PRIMARY | 4       | const |    1 | Using where |
+----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+

虽然这个查询和情况五中功能相同，但是由于使用了函数left，则无法为title列应用索引，而情况五中用LIKE则可以。再如：

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EXPLAIN SELECT * FROM employees.titles WHERE emp_no - 1='10000';
+----+-------------+--------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | titles | ALL  | NULL          | NULL | NULL    | NULL | 443308 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+--------+-------------+

显然这个查询等价于查询emp_no为10001的函数，但是由于查询条件是一个表达式，MySQL无法为其使用索引。看来MySQL还没有智能到自动优化常量表达式的程度，因此在写查询语句时尽量避免表达式出现在查询中，而是先手工私下代数运算，转换为无表达式的查询语句。