https://www.mongodb.com/docs/manual/reference/operator/projection/elemMatch/

If there are indexes for that field, will the $elemMatch operator use it during $projection?

CodePudding user response：

No, a $project stage will not make use of an index.

Indexes are used for locating documents that match a query without needing to read the entire collection.

A $project stage is used to mutate documents already in the pipeline, so it will never retrieve documents from the collection.

If the pipeline starts with a $project stage, the query executor will add an implicit collection scan (effectively a {$match: {}}) before the projection.