sql-server - 按顺序插入更快

Question

我有以下查询：

 INSERT INTO table(field, field1)
 SELECT value, value1  FROM table2 e 
 ORDER BY value

这比这个花费更少的时间：

 INSERT INTO table(field, field1)
 SELECT value, value1  FROM table2 e 

有谁知道为什么？

第二个的执行计划显示sql无论如何都会执行“order by”操作，但性能不如我的

Answer 1

Insert performance depends on how many indexes you have and on what columns. If there is a clustered index on table.field inserting unsorted values is quite expensive (values not sorted by field).

Answer 2

Do you have any nonclustered index on the value column in the table table2? Do you have clustered index on table on value clause? I could see two possible reason for this.

1.There is some kind of nonclustered index on column value so that optimizer picks this index and avoids sorts (it could be a covering index as well, in this case it will be very fast). The reason why the query without any order by did not pick that index is because it is a simple query and no optimization happened and it did a clustered index or table scan and then sorted the data and it caused the performance degraded as compared to order by. This is the most likely reaosn.

1. The other reason could be that while inserting the data it inserts the data as it get and then if the data is ordered and same clustered index on order by column. There are no page splits and things will be fine.But if there is no sort then values will be inserted randomly an dcould cause page splits which slightly degrades performance. However, OP has mentioned that optimizer has done a sort before inserting that means this scenario is not applicable here.