TOP N 和SET ROWCOUNT N 哪个更快？ -pc6资讯

您的位置：首页 → 技术开发 → 数据库技巧 → TOP N 和SET ROWCOUNT N 哪个更快？

TOP N 和SET ROWCOUNT N 哪个更快？ 时间：2004/12/2 0:59:00来源：本站整理作者：蓝点我要评论(0): 懒得翻译了，大意：

在有合适的索引的时候，Top n和set rowcount n是一样快的。但是对于一个无序堆来说，top n更快。

原理自己看英文去。

Q. Is using the TOP N clause faster than using SET ROWCOUNT N to return a specific number of rows from a query?

A. With proper indexes, the TOP N clause and SET ROWCOUNT N statement are equally fast, but with unsorted input from a heap, TOP N is faster. With unsorted input, the TOP N operator uses a small internal sorted temporary table in which it replaces only the last row. If the input is nearly sorted, the TOP N engine must delete or insert the last row only a few times. Nearly sorted means you're dealing with a heap with ordered inserts for the initial population and without many updates, deletes, forwarding pointers, and so on afterward.

A nearly sorted heap is more efficient to sort than sorting a huge table. In a test that used TOP N to sort a table with the same number of rows but with unordered inserts, TOP N was not as efficient anymore. Usually, the I/O time is the same both with an index and without; however, without an index SQL Server must do a complete table scan. Processor time and elapsed time show the efficiency of the nearly sorted heap. The I/O time is the same because SQL Server must read all the rows either way.

文章评论

查看所有0条评论>>

热门文章 没有查询到任何记录。