索引的数据结构及算法原理--索引使用策略及优化（下）

情况四：查询条件没有指定索引第一列

1. EXPLAIN SELECT * FROM employees.titles WHERE from_date='1986-06-26';
2. +----+-------------+--------+------+---------------+------+---------+------+--------+-------------+
3. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
4. +----+-------------+--------+------+---------------+------+---------+------+--------+-------------+
5. | 1 | SIMPLE | titles | ALL | NULL | NULL | NULL | NULL | 443308 | Using where |
6. +----+-------------+--------+------+---------------+------+---------+------+--------+-

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

1. EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001' AND title LIKE 'Senior%';
2. +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
3. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
4. +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
5. | 1 | SIMPLE | titles | range | PRIMARY | PRIMARY | 56 | NULL | 1 | Using where |
6. +----+-------------+--------+-------+---------------+---------+---------+------+---

此时可以用到索引，但是如果通配符不是只出现在末尾，则无法使用索引。（原文表述有误，如果通配符%不出现在开头，则可以用到索引，但根据具体情况不同可能只会用其中一个前缀）

情况六：范围查询

1. EXPLAIN SELECT * FROM employees.titles WHERE emp_no < '10010' and title='Senior Engineer';
2. +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
3. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
4. +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
5. | 1 | SIMPLE | titles | range | PRIMARY | PRIMARY | 4 | NULL | 16 | Using where |
6. +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+

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。

1. EXPLAIN SELECT * FROM employees.titles
2. WHERE emp_no < 10010'
3. AND title='Senior Engineer'
4. AND from_date BETWEEN '1986-01-01' AND '1986-12-31';
5. +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
6. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
7. +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
8. | 1 | SIMPLE | titles | range | PRIMARY | PRIMARY | 4 | NULL | 16 | Using where |
9. +----+-------------+--------+-------+---------------+---------+---------+------+------+------------

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：

1. EXPLAIN SELECT * FROM employees.titles
2. WHERE emp_no BETWEEN '10001' AND '10010'
3. AND title='Senior Engineer'
4. AND from_date BETWEEN '1986-01-01' AND '1986-12-31';
5. +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
6. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
7. +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
8. | 1 | SIMPLE | titles | range | PRIMARY | PRIMARY | 59 | NULL | 16 | Using where |
9. +----+-------------+--------+-------+---------------+---------+---------+------+------+-----------

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：

1. EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001' AND left(title, 6)='Senior';
2. +----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+
3. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
4. +----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+
5. | 1 | SIMPLE | titles | ref | PRIMARY | PRIMARY | 4 | const | 1 | Using where |
6. +----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+

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：

1. EXPLAIN SELECT * FROM employees.titles WHERE emp_no - 1='10000';
2. +----+-------------+--------+------+---------------+------+---------+------+--------+-------------+
3. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
4. +----+-------------+--------+------+---------------+------+---------+------+--------+-------------+
5. | 1 | SIMPLE | titles | ALL | NULL | NULL | NULL | NULL | 443308 | Using where |
6. +----+-------------+--------+------+---------------+------+---------+------+--------+-------------+

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。