2011-09-28Queries Without Table Access

The previous installment demonstrated how to use an index to cluster table rows together. The examples showed that even “anywhere” LIKE expressions can be tuned when the column is put into the index to avoid the table access.

Todays installment extends this concept and shows queries that don’t need to access the table at all. It’s about the so-called index-only scan, or, less descriptive, covering index.