What is a covered query?

Short answer

A query answered entirely from an index: every filter and projected field is in that index, so the server never fetches the document. In explain you see IXSCAN without a FETCH.

Why?

You usually have to exclude _id from the projection unless _id is in the index. Covered queries are worth it on hot, skinny reads. Widening an index only to cover a query is a write-cost trade.

Interview tip

Mention IXSCAN without FETCH. That is the explain fingerprint.

Common mistake

Calling any IXSCAN a covered query. IXSCAN plus FETCH still touches documents.