sql-server - 什么是伪合并连接？

Question

这是 Microsoft TechNet 的描述解释跟踪标志 342(添加重点):

Disables the costing of pseudo-merge joins, thus significantly reducing time spent on the parse for certain types of large, multi-table joins. One can also use SET FORCEPLAN ON to disable the costing of pseudo-merge joins because the query is forced to use the order specified in the FROM clause.

你们有人知道什么是伪合并连接吗？据我所知，SQL Server 有 3 种连接算法(Nest Loop Join、Merge Join 和 Hash Join - 其中包括 Bitmap Join)。那么什么是伪合并连接，它与常规合并连接或任何其他连接有什么区别？

Answer 1

我知道这是一个老问题，但我会尽力回答它。

伪合并不是一种用作T-SQL语言运算符的Join，我对微软的解释explanation使用Trace Flag 342如下:

Disables the costing of pseudo-merge joins, thus significantly reducing time spent on the parse for certain types of large, multi-table joins.

伪合并这个概念表示查询优化器正在尝试计算更好的查询执行计划，试图获得连接多个表的最佳方式。

One can also use SET FORCEPLAN ON to disable the costing of pseudo-merge joins because the query is forced to use the order specified in the FROM clause.

此选项prevents the optimizer from trying to calculate并简单地执行查询中列出的连接。

关于 SET FORCEPLAN ON 的文章供引用。