实际上,您可以把索引理解为一种特殊的目录。微软的SQL SERVER提供了两种索引:聚集索引(clustered index,也称聚类索引、簇集索引)和非聚集索引(nonclustered index,也称非聚类索引、非簇集索引)。下面,我们举例来说明一下聚集索引和非聚集索引的区别: 其实,我们的汉语字典的正文本身就是一个聚集索引。比如,我们要查"
NonclusteredIndex 与 clusteredIndex不一样之处在于,NonclusteredIndex是另外单独进行数据表格化,相当于新建一个中间表用于 存储index 列及物理表的位置。如果Index中包括了所有的查询及检索对象,其执行速度会显著增加。 详细可以参照: https://docs.microsoft.com/en-us/sql/relational-databases/indexes/clustered-and-...
这里因为是建了聚集索引,表数据存储在索引的叶子节点,所以应用的clustered index scan,读取的行数还是100W,所有实际执行的行数为1行。 非聚集索引下WHERE查询 --在NAME字段上建立非聚集索引。 CREATE NONCLUSTERED INDEX IX_EMP_NAME ON EMPLOYEES(NAME) -- 再次执行WHERE查询并含实际执行计划。 SELECT * FROM EMP...
1. 什么是聚合索引(clustered index) / 什么是非聚合索引(nonclustered index)? 2. 聚合索引和非聚合索引有什么区别? 深入浅出理解索引结构 实际上,您可以把索引理解为一种特殊的目录。微软的SQL SERVER提供了两种索引:聚集索引(clustered index,也称聚类索引、簇集索引)和非聚集索引(nonclustered index,也称非聚类...
KEY, Database Engine automatically creates a clustered index, unless a clustered index already exists. When you try to enforce a PRIMARY KEY constraint on an existing table and a clustered index already exists on that table, SQL Server enforces the primary key using a nonclustered index....
When you create a UNIQUE constraint, a unique nonclustered index is created to enforce a UNIQUE constraint by default. You can specify a unique clustered index if a clustered index on the table doesn't already exist. An index created as part of the constraint is automatically given the same...
a nonclustered index has a single partition. When a nonclustered index has multiple partitions, each partition has a B-tree structure that contains the index rows for that specific partition. For example, if a nonclustered index has four partitions, there are four B-tree structures, with one...
When you create a UNIQUE constraint, a unique nonclustered index is created to enforce a UNIQUE constraint by default. You can specify a unique clustered index if a clustered index on the table doesn't already exist. An index created as part of the constraint is automatically given the same...
a nonclustered index has a single partition. When a nonclustered index has multiple partitions, each partition has a B-tree structure that contains the index rows for that specific partition. For example, if a nonclustered index has four partitions, there are four B-tree structures, with one...
Each disk-based nonclustered index in your database takes up space, adds overhead for inserts, updates, and deletes, and might require maintenance. For these reasons, it's a best practice to review all the missing index requests for a table and the existing indexes on a table before...