Answer
Bookmark lookup occurs when SQL uses a non-clustered index but must fetch extra columns from the base table.
This becomes slow when many rows match, increasing I/O and reducing performance.
Quick Answer
Bookmark lookup happens when a non-clustered index finds the matching row keys, but the query needs columns not in the index — so SQL Server does an extra lookup into the clustered index to get them. Each lookup is a random I/O. With many rows, this becomes very expensive. Fix: use a covering index.
S
SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ