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?
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Bear Bile Farming is Torture
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I agree with [@Joe's answer](https://stackoverflow.com/a/74905673/20042973). But I'm a little curious about the specifics of your question. In what way(s) were you thinking that the index may be used there? – user20042973 Dec 24 '22 at 14:13
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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.
Joe
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