MySQL max函数优化

测试环境：

测试Sql：

select max(sscode) from	sm_sku_show where del_flag = "0";
select sscode from sm_sku_show where del_flag = "0" order by sscode desc limit 1;

测试耗时：

[SQL]
select max(sscode) from	sm_sku_show where del_flag = "0";
受影响的行: 0
时间: 19.503s

[SQL]
select sscode from sm_sku_show where del_flag = "0" order by sscode desc limit 1;
受影响的行: 0
时间: 0.001s

测试结果：

+---------------------+
| max(sscode)         |
+---------------------+
| SS20180120101925999 |
+---------------------+
1 row in set

+---------------------+
| sscode              |
+---------------------+
| SS20180120101925999 |
+---------------------+
1 row in set

结论：

max() 函数的需扫描where条件过滤后的所有行。

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