Moved the element to a cached property, along with its permission checks, to remove two duplicate queries.
Removed the prefetch for element__image__server when not using recursive, since the element is never returned.
Changed the element filter when using recursive to use a UNION on the element IDs from ElementPath + the ID of the current element.
This no longer loads every single element in RAM.
This does not use Q(element=element) | Q(element__in=…) because that makes PostgreSQL ignore almost every index and start using nested loops.
This does not use get_descending, because it uses some unnecessary ordering and deduplication.
This uses ElementPath.element_id and not Element.id to avoid an unnecessary join.
This uses UNION ALL and not just UNION, to avoid an unnecessary cycle of sorting and deduplicating IDs.
Some benchmarks on the strange id::varchar sort did not show any difference with a normal id sort, so I left it there, but it does not seem to be as "inefficient" as what the comments say.
This makes recursive ListTranscriptions up to 10 times faster on top-level folders of random corpora with many transcriptions.
