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Data & Analytics · Intermediate

Nonclustered Index

A separate index structure containing key values and pointers to the data rows; a table can have many.

Definition

A nonclustered index stores sorted key columns plus a row locator (the clustered key or a RID for a heap). If the query needs columns not in the index, the engine performs key lookups, which become expensive for many rows.

Example

CREATE NONCLUSTERED INDEX IX_Cust_Email ON Customers (Email);

Exam tip

Frequent key lookups in a plan suggest adding INCLUDE columns to make the index covering.

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