慎用mysql max()函数，可通过走索引替换

mysql max() 函数的需扫描where条件过滤后的所有行：

在测试环境中重现：

测试版本：Server version:         5.1.58-log MySQL Community Server (GPL)

testtable表中的索引

mysql> show index from testtable;
+-----------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table     | Non_unique | Key_name   | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-----------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| testtable |          0 | PRIMARY    |            1 | id          | A         |           2 |     NULL | NULL   |      | BTREE      |         |
| testtable |          1 | key_number |            1 | number      | A         |           2 |     NULL | NULL   | YES  | BTREE      |         |
+-----------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

对比的sql为：

 select sql_no_cache  max(id) from testtable where number=98;
 select sql_no_cache id from testtable where number=98 order by id desc limit 1;

查看执行计划：

mysql> explain select sql_no_cache  max(id) from testtable where number=98;
+----+-------------+-----------+------+---------------+------------+---------+-------+------+--------------------------+
| id | select_type | table     | type | possible_keys | key        | key_len | ref   | rows | Extra                    |
+----+-------------+-----------+------+---------------+------------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | testtable | ref  | key_number    | key_number | 5       | const |    4 | Using where; Using index |
+----+-------------+-----------+------+---------------+------------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)


mysql> explain select sql_no_cache id from testtable where number=98 order by id desc limit 1;
+----+-------------+-----------+------+---------------+------------+---------+-------+------+--------------------------+
| id | select_type | table     | type | possible_keys | key        | key_len | ref   | rows | Extra                    |
+----+-------------+-----------+------+---------------+------------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | testtable | ref  | key_number    | key_number | 5       | const |    4 | Using where; Using index |
+----+-------------+-----------+------+---------------+------------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)
执行计划显示完全一样。

其中，number为98 对应的记录有4行：

mysql> select count(*) from testtable where number=98;

+----------+
| count(*) |
+----------+
|        4 |
+----------+
1 row in set (0.00 sec)

执行前查看innodb_rows_read

#innodb_rows_read  从InnoDB表读取的行数

mysql> show global status like 'innodb_rows_read';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| Innodb_rows_read | 1022  |
+------------------+-------+
1 row in set (0.00 sec)

执行sql1
mysql> select sql_no_cache  max(id) from testtable where number=98;
+---------+
| max(id) |
+---------+
|      13 |
+---------+

1 row in set (0.00 sec)

执行后查看innodb_rows_read，发现innodb_rows_read增加了4，即number为98 对应的记录有4行
mysql> show global status like 'innodb_rows_read';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| Innodb_rows_read | 1026  |
+------------------+-------+
1 row in set (0.00 sec)

执行sql2
mysql> select sql_no_cache id from testtable where number=98 order by id desc limit 1;
+----+
| id |
+----+
| 13 |
+----+
1 row in set (0.00 sec)

执行后查看innodb_rows_read，发现innodb_rows_read增加了1 
mysql> show global status like 'innodb_rows_read';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| Innodb_rows_read | 1027  |
+------------------+-------+
1 row in set (0.00 sec)

测试得出：

 select sql_no_cache  max(id) from testtable where number=98;

需要读取 number=98 的所有行，才能得到最大的id

 select sql_no_cache id from testtable where number=98 order by id desc limit 1;

由于id是主键，number是第二索引，只需扫描1行即可得到最大的id

请慎用max()函数，特别是频繁执行的sql，若需用到可转化为测试中的  order by id desc limit 1