In order to get a list of all pre-image collections present in config.system.preimages we are performing something similar to DISTINCT_SCAN on _id.nsUUID.

The problem is that right now DISTINCT_SCAN falls back to a full collection scan since there are no indexes available for the collection and we are scanning a subfield of _id.

We can work around this in our use-case since we know the format of _id and that nsUUID is the first field of the identifier. As a result we can safely enumerate them using simple RecordCursor::seekNear. Ideally we would prefer to use DISTINCT_SCAN.