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懒得翻译了，大意： 在有合适的索引的时候，Top n和set rowcount n是相同快的。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. 返回类别: 教程 上一教程: 保护SQL Server的十个步骤 下一教程: 如何识别真实和自动创建的索引？ 您可以阅读与"TOP N 和SET ROWCOUNT N 哪个更快"相关的教程: · TOPN 子句与SET ROWCOUNTN 之对比 · MYSQL中如何实现TOP N及M至N段的记录查询？ · 讨论: SELECT TOP N问题 (2) · 如何让你的SQL运行得更快 · 讨论: SELECT TOP N 问题 |
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