陷阱~SQL全表扫描与聚集索引扫描-Mysql Tutorial-php.cn

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陷阱~SQL全表扫描与聚集索引扫描

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Jun 07, 2016 pm 05:39 PM

scanningindextrap

SqlServer中在查询时，我们为了优化性能，通常会为where条件的字段建立索引，如果条件比较固定还会建立组合索引，接下来，我们来看一下索引与查询的相关知识及相关陷阱。ＳＱＬ表自动为主键加聚集索引的猜想我认为应该是对查询的优化，因为如果聚集（最多

SqlServer中在查询时，，我们为了优化性能，通常会为where条件的字段建立索引，如果条件比较固定还会建立组合索引，接下来，我们来看一下索引与查询的相关知识及相关陷阱。

ＳＱＬ表自动为主键加聚集索引的猜想

我认为应该是对查询的优化，因为如果聚集（最多只能有一个）索引的话，在ＳＱＬ查询时，将进行全表扫描，反之，进行聚集索引（Ｂ树结构）扫描，这将大大加快检索的速度；有时，我们感觉对主键（自增的）加聚集索引没什么用，所以就把它改为非聚集索引，但当我们为表的其它字段索引后，发展查询时没有按着索引检索，而是全表扫描，这是为何？原因是表中没有任何字段是聚集的索引，所以你还必须为这个索引加上聚集的，sqlserver怕我们忘记加“聚集”索引，所以当我们在设置主键时，它帮助我们自动为主键加上聚集索引，这是那个原因。

对于存在聚集索引的表，在查询计划中可以观察到它是走聚集索引的

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