Answer
A covering index includes all columns a query needs. This eliminates key lookups and reduces I/O. Covering indexes provide dramatic performance improvements in OLTP systems by minimizing data access and improving plan efficiency.
Quick Answer
A covering index includes all columns a query needs — so SQL Server can answer it entirely from the index without touching the base table. This eliminates key lookups, reduces logical reads, and often dramatically cuts query time. Add SELECT columns via INCLUDE to cover without making the index key unnecessarily wide.
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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