Answer
Key lookup happens when a non-clustered index finds a row but SQL needs extra columns. It causes many random I/O operations and slows queries.
Quick Answer
Key lookup (aka Bookmark Lookup) happens when a non-clustered index finds the row, but the query needs extra columns not in the index — SQL Server fetches those from the clustered index. Each lookup is a random I/O. With thousands of rows, this stacks up. Adding INCLUDE columns to the index eliminates it.
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.
Want to bookmark, take notes, or join discussions?
Sign in to access all features and personalize your learning experience.
Source: SugharaIQ