Is there a better way of doing a query like this:
FROM (SELECT DISTINCT DocumentId, DocumentSessionId
FROM DocumentOutputItems) AS internalQuery
If you are trying to improve performance, you could try creating a persisted computed column on either a hash or concatenated value of the two columns.
Once it is persisted, provided the column is deterministic and you are using "sane" database settings, it can be indexed and / or statistics can be created on it.
I believe a distinct count of the computed column would be equivalent to your query.