An index, when used for an index-only scan.
Links
Book-Section: Index-Only Scan: Avoiding Table Access
Glossary: Index-Only Scan — A better name for Covering Index
Covering Index
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Markus Winand provides insights into SQL and shows how different systems support it at modern-sql.com. Previously he made use-the-index-luke.com, which is still actively maintained. Markus can be hired as trainer, speaker and consultant via winand.at.
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