TOP N 和SET ROWCOUNT N 哪个更快[MSSQL防范]

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 wa
　　以上是“TOP N 和SET ROWCOUNT N 哪个更快[MSSQL防范]”的内容，如果你对以上该文章内容感兴趣，你可以看看七道奇为您推荐以下文章：