by Markus Winand.

Index-Only Scan


The Index-Only Scan is an index scan without subsequent table access—hence, accessing the index only.

All recent versions of Oracle, SQL Server and MySQL support index-only scans. The PostgreSQL database supports index-only scans since release 9.2.

A query can be executed with an Index-Only Scan if all queried data is available in the index. That means that even the columns that appear in the select clause only, must be included in the index.

The performance advantage of an index-only scans depends on the number of accessed rows and the index clustering factor.

Links

About the Author

Photo of Markus Winand

Markus Winand teaches efficient SQL—inhouse and online. He minimizes the development time using modern SQL and optimizes the runtime with smart indexing—for that he also published the book SQL Performance Explained.

Markus’ Book

Cover of “SQL Performance Explained”: Squirrel running on grass

The essence of SQL tuning in 200 pages.

Buy from Markus
(paperback and/or PDF)

Buy from Amazon
(paperback only)

Hire Markus

…to answer your current SQL questions.

The quick and easy way to benefit from his extensive knowledge and experience. Learn more»

“Use The Index, Luke!” by Markus Winand is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 Unported License.
Legal | Contact | NO WARRANTY | Trademarks | Privacy | CC-BY-NC-ND 3.0 license