For details about index key limits, see Maximum capacity specifications for SQL Server. Both clustered and nonclustered indexes can be unique. With a unique index, no two rows can have the same value for the index key. Otherwise, the index isn't unique and multiple rows can share the same...
SQL Server automatically creates indexes when PRIMARY KEY and UNIQUE constraints are defined on table columns. For example, when you create a table with a UNIQUE constraint, Database Engine automatically creates a nonclustered index. If you configure a PRIMARY KEY, Database Engine automatically...
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"...
CREATEUNIQUENONCLUSTERED INDEX IX_NonClustered_EmployeeONdbo.Employee(EmpContactNumber); 1 在我们解释这个索引之前,重新运行 SELECT 语句并查看实际的执行计划: 在这个执行计划中,我们可以看到两个组件: 索引查找(非聚集) - Index Seek(NonClustered) 键查找(集群) - Key Lookup(Clustered) 要了解这些组件,我们...
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...
1. 什么是聚合索引(clustered index) / 什么是非聚合索引(nonclustered index)? 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...
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)
SQL USEAdventureWorks2022; GO-- Find an existing index named IX_ProductVendor_VendorID and delete it if found.IF EXISTS (SELECTnameFROMsys.indexesWHEREname= N'IX_ProductVendor_VendorID')DROPINDEXIX_ProductVendor_VendorIDONPurchasing.ProductVendor; GO-- Create a nonclustered index called IX_Produc...
Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance SQL database in Microsoft Fabric Follow these steps to estimate the amount of space that is required to store a nonclustered index: Calculate variables for use in steps 2 and 3. Calculate the space used to store index ...
Because data is inserted without enforcing a strict order, the insert operation is usually faster than the equivalent insert into a clustered index. If the heap's data will be read and processed into a final destination, it may be useful to create a narrow nonclustered index that covers the...