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Data & Analytics · Advanced

Key Lookup

An execution-plan operation in which the engine uses a nonclustered index and then fetches the remaining columns from the clustered index for each row.

Definition

Key lookups (bookmark lookups, or RID lookups on heaps) are cheap for a few rows but costly for many. The usual fix is a covering index with INCLUDE columns, or reducing the selected columns.

Exam tip

A plan showing Index Seek plus Key Lookup on many rows is the textbook sign that an index should be made covering.

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