What is a covered query?

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Accepted Answer

A covered query is one that can be answered entirely from an index — MongoDB never reads the actual document. Drastically faster. Conditions — all fields in the query and projection must be in the index, and the projection must exclude _id (or include it explicitly via the index). Verify with explain() — look for IXSCAN with no FETCH stage. Especially useful — for high-frequency lookups, count queries, and pagination keys. Trade-off — bigger compound indexes slow writes and use more RAM. Covered query = all filter + projection fields in the index + _id excluded. totalDocsExamined: 0 in explain proves it is covered. The _id exclusion requirement is the most commonly missed detail. Covered queries are the fastest possible queries.