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
因为往往min()或者max()函数往往会造成全表扫描