Covering Index
An index covers a query when every referenced column is present in the index (as a key or INCLUDE/stored column), letting the engine answer with an index-only scan and never touch the table heap. This trades write amplification and index size for eliminating the random heap lookups that dominate many read plans.
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An index covers a query when every referenced column is present in the index (as a key or INCLUDE/stored column), letting the engine answer with an index-only scan and never touch the table heap. This trades write amplification and index size for eliminating the random heap lookups that dominate many read plans.
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