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糟糕的sql语句
同事传过来一个sql,说此sql运行期间大量占用服务器磁盘空间,大约将近10个G的空间,效率超慢
SELECT MIN(order_id), MAX(order_id) FROM (select *,date(order_creation_date) create_date from orders where (1 = 1) ) AS t1
1 又是一个糟糕的sql,select * from table_name 这种格式的sql是绝对不允许的
2 刚开始,没看明白这个sql是要干嘛的,为什么要从这个表里查询数据呢?select *,date(order_creation_date) create_date from orders where (1 = 1),作为嵌套表。
3 在查询过程中,为什么会占用磁盘空间
中间产生了临时表,内存中放不下这么多数据,把数据拷贝到磁盘中,一般在 /tmp 下面 产生类似 #sql_***.MYD,#sql_***.MYI 这样的文件,其实就是一个myisam表。
这样拷贝的过程浪费时间,影响性能。
咨询同事,只是想查询出order_id的最大值,最小值。 看了一下orders表结构,orders的order_id是自增主键,那就简单了, 直接改写如下
select max(order_id),min(order_id) from orders
order_id 是自增主键,上面已经建了索引,而且是有序的,因此对于max min这种复合函数求最大最少值非常的快。
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