SQL Server中的Nested Loops 1. 解释什么是SQL Server中的Nested Loops Nested Loops(嵌套循环)是SQL Server中用于执行表连接操作的一种基本方法。它采用两层循环的方式,将一张表作为外部表(Outer Table),另一张表作为内部表(Inner Table),然后对外部表的每一行,在内部表中寻找匹配的行。 2. 描述Nested Loops在...
因为nested loops join的复杂度是与驱动表大小和内部表大小乘积成比例的,所以在驱动表比较小 的情况下性能比较好。内部表不需要很小,但是如果非常大的话,在具有高选择性的连接列上建立索引将很有帮助。 一些情况下,Sql Server只能使用nested loops join算法,比如Cross join和一些复杂的cross applies,outer applies, ...
In some cases, a nested loops join is the only join algorithm that SQL Server can use. SQL Server must use a nested loops join for cross join as well as some complex cross applies and outer applies. Moreover, with one exception (for full outer join), a nested loops join is the only...
1.嵌套循环连接(Nested Loops)适用范围 两个表, 一个叫外部表, 一个叫内部表. 如果外部输入非常小,而内部输入非常大并且已预先建立索引,那么嵌套循环联接将特别有效率。 关于连接时哪个表为outer表,哪个为inner表,我发现sql server会自动给你安排,和你写的位置无关,它自动选择数据量小的表为outer表, 数据量大...
Nested Loops Join(嵌套连接) Sql Server支持三种物理连接:nested loops join,merge join和hash join.这篇文章,我将描述nested loops join (或者简称为NL)。 基本算法 最简单的情况是,nested loop会以连接谓词为条件取出一张表里的每一行(称为外部表)
Nested Loops Join(嵌套连接) Sql Server支持三种物理连接:nested loops join,merge join和hash join.这篇文章,我将描述nested loops join (或者简称为NL)。 基本算法 最简单的情况是,nested loop会以连接谓词为条件取出一张表里的每一行(称为外部表)
SQL Server supports three physical join operators: nested loops join, merge join, and hash join. In this post, I’ll describe nested loops join (or NL join for short). The basic algorithm In its simplest form, a nested loops join compares each row from one table (known as the outer ta...
SQL Server supports three physical join operators: nested loops join, merge join, and hash join. In this post, I’ll describe nested loops join (or NL join for short). The basic algorithm In its simplest form, a nested loops join compares each row from one table (known as the outer ...
因为普通Nested-Loop一次只将一行传入内层循环, 所以外层循环(的结果集)有多少行, 内存循环便要执行多少次.在内部表的连接上有索引的情况下,其扫描成本为O(Rn),若没有索引,则扫描成本为O(Rn*Sn)。如果外部表有很多记录,则Nested-Loops Join会扫描内部表很多次,执行效率非常差。
First published on MSDN on Jul 26, 2006 SQL Server supports three physical join operators: nested loops join, merge join, and hash join.