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。