今天在review 一个SQL的时候，发现即使在列前面有函数操作，查询也能够使用到索引，如下：（OLD）
1 mysql> desc SELECT sql_no_cache COUNT(1) FROM Appointment WHERE YEAR(ScheduledDate)=YEAR(NOW()) AND MONTH(ScheduledDate)=MONTH(NOW());
+----+-------------+-------------+-------+---------------+---------------+---------+------+---------+--------------------------+ 2 | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | 3 +----+-------------+-------------+-------+---------------+---------------+---------+------+---------+--------------------------+ 4 | 1 | SIMPLE | Appointment | index | NULL | ScheduledDate | 9 | NULL | 1812978 | Using where; Using index | 5 +----+-------------+-------------+-------+---------------+---------------+---------+------+---------+--------------------------+ 6 1 row in set (0.00 sec)
用的是全索引扫描(type=index)，用到了索引(key=ScheduledDate),并且用的是覆盖索引(Using index).看起来这个SQL没有问题，唯一不足之处在1)type列，INDEX是效率很差的连接类型，尽优于全表扫描(full). 2)扫描的行数太多(row=1812978).
按照最常见的优化方法改写SQL后的执行计划如下：（NEW）
1 mysql> desc SELECT sql_no_cache COUNT(1) FROM Appointment WHERE ScheduledDate > DATE_FORMAT(NOW(),'%Y-%m-01 00:00:00') AND ScheduledDate < DATE_FORMAT(DATE_ADD(NOW(),INTERVAL 1 MONTH),'%Y-%m-01 00:00:01'); 2 +----+-------------+-------------+-------+---------------+---------------+---------+------+--------+--------------------------+ 3 | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | 4 +----+-------------+-------------+-------+---------------+---------------+---------+------+--------+--------------------------+ 5 | 1 | SIMPLE | Appointment | range | ScheduledDate | ScheduledDate | 9 | NULL | 727166 | Using where; Using index | 6 +----+-------------+-------------+-------+---------------+---------------+---------+------+--------+--------------------------+ 7 1 row in set (0.00 sec)
连接方式变成了范围查找(type=range),扫描的行数也减少了很多(rows=727166) 。
2个SQL的实际执行效率，在结果集很大的时候，还是存在比较大的差别
| 1 | 2 | 3 | 4 | 5 | |
| OLD | 2.75 | 2.51 | 2.35 | 2.56 | 2.77 |
| NEW | 0.49 | 0.65 | 0.50 | 0.54 | 0.60 |
比较两个查询的平均执行时间OLD=(2.75+2.51+2.35+2.56+2.77)/5=2.588，NEW=0.556，效率相差 2.588/0.556=4.65.
分析一下1）为什么使用到函数了，还会用到索引？ 2）同样是使用了覆盖索引，为什么会出现严重的性能偏差？
这里举的例子有特殊性，select count(1),获取全部满足条件的总记录数，只需要扫描符合条件的索引页，就能够得到正确的结果集，不需要回表取数据。这是问题1的解释。在OLD里面，需要扫描了全部的索引页，然后对里面的数据进行计算比较。在NEW里面，只需要扫描少数的索引页，就能够得到结果集。这些差别在row里面体现了出来。这是问题2的解释。
在非覆盖索引下，一般的情况查询应该是这样的：
1 mysql> desc SELECT sql_no_cache * FROM Appointment WHERE YEAR(ScheduledDate)=YEAR(NOW()) AND MONTH(ScheduledDate)=MONTH(NOW()); 2 +----+-------------+-------------+------+---------------+------+---------+------+---------+-------------+ 3 | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | 4 +----+-------------+-------------+------+---------------+------+---------+------+---------+-------------+ 5 | 1 | SIMPLE | Appointment | ALL | NULL | NULL | NULL | NULL | 1541817 | Using where | 6 +----+-------------+-------------+------+---------------+------+---------+------+---------+-------------+ 7 1 row in set (0.00 sec)
此时只通过索引是无法检索出结果集，所以使用了全表扫描。
优化后的执行计划是这样的：
1 mysql> desc SELECT sql_no_cache * FROM Appointment WHERE ScheduledDate > DATE_FORMAT(NOW(),'%Y-%m-01 00:00:00') AND ScheduledDate < DATE_FORMAT(DATE_ADD(NOW(),INTERVAL 1 MONTH),'%Y-%m-01 00:00:01'); 2 +----+-------------+-------------+------+---------------+------+---------+------+---------+-------------+ 3 | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | 4 +----+-------------+-------------+------+---------------+------+---------+------+---------+-------------+ 5 | 1 | SIMPLE | Appointment | ALL | ScheduledDate | NULL | NULL | NULL | 1541817 | Using where | 6 +----+-------------+-------------+------+---------------+------+---------+------+---------+-------------+ 7 1 row in set (0.00 sec)
这里还是使用了全表扫描。MYSQL中查询返回的结果行占表总行数超过15%（测试下来是15%左右是优化器走索引还是全部扫描的分割线，手册里写的是30%）时，就会选择全表扫描。在这个案例里，上述条件返回337711，总记录数1698272，337711/1698272=0.1989，超过门限值，所以用了全表扫描。缩小结果集
1 mysql> SELECT sql_no_cache count(*) FROM Appointment WHERE ScheduledDate > DATE_FORMAT(NOW(),'%Y-%m-%d 00:00:00') AND ScheduledDate < DATE_FORMAT(DATE_ADD(NOW(),INTERVAL 1 day),'%Y-%m-%d 00:00:01'); 2 +----------+ 3 | count(*) | 4 +----------+ 5 | 10169 | 6 +----------+ 7 1 row in set (0.01 sec) 8 9 mysql> desc SELECT sql_no_cache * FROM Appointment WHERE ScheduledDate > DATE_FORMAT(NOW(),'%Y-%m-%d 00:00:00') AND ScheduledDate < DATE_FORMAT(DATE_ADD(NOW(),INTERVAL 1 day),'%Y-%m-%d 00:00:01'); 10 +----+-------------+-------------+-------+---------------+---------------+---------+------+-------+-------------+ 11 | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | 12 +----+-------------+-------------+-------+---------------+---------------+---------+------+-------+-------------+ 13 | 1 | SIMPLE | Appointment | range | ScheduledDate | ScheduledDate | 9 | NULL | 20924 | Using where | 14 +----+-------------+-------------+-------+---------------+---------------+---------+------+-------+-------------+ 15 1 row in set (0.00 sec)
这里执行计划和预期相同。