If our query includes the shard key or a prefix of the shard key, mongos will perform a targeted operation, only querying the shards that hold the keys that we are looking for.

For example, with a composite shard key of {_id, email, address} on our collection User, we can have a targeted operation with any of the following queries:

> db.User.find({_id: 1})
> db.User.find({_id: 1, email: '[email protected]'})
> db.User.find({_id: 1, email: '[email protected]', address: 'Linwood Dunn'})

All three of them are either a prefix (the first two) or the complete shard key.

On the other hand, a query on {email, address} or {address} will not be able to target the right shards, resulting in a broadcast operation.

A broadcast operation is any operation that doesn't include the shard key or a prefix of the shard key and results in mongos querying every shard and gathering results from them. It's also known as a scatter-and-gather operation or a fanout query.