sql - What is a Covered Index? -

i've heard term covered index in database discussion - mean?

a covering index index contains of, , possibly more, columns need query.

for instance, this:

select * tablename criteria 

will typically use indexes speed resolution of rows retrieve using criteria, go full table retrieve rows.

however, if index contained columns column1, column2 , column3, sql:

select column1, column2 tablename criteria 

and, provided particular index used speed resolution of rows retrieve, index contains values of columns you're interested in, won't have go table retrieve rows, can produce results directly index.

this can used if see typical query uses 1-2 columns resolve rows, , typically adds 1-2 columns, beneficial append columns (if they're same over) index, query processor can index itself.

here's article: index covering boosts sql server query performance on subject.