possible fix for #3372 - user export timeouts

[hughrun]

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• [x] Bug Fix
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Description

This definitely needs to be tested on a large DB but I believe it may fix the timeouts b.s. gets when running user exports.

Instead of a gigantic single DB query with heaps of joins, we instead just do a series of simple queries and then use union() to pull them into a de-duped queryset.

If I understand the results from explain() correctly, this is a massive reduction in DB work:

Unique (cost=195899.15..198201.71 rows=11808 width=19220)

vs

Unique (cost=150.28..153.44 rows=16 width=19220)

• Related Issue #
• Closes #3372

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[mouse-reeve]

I tried this with the query time test system in #3380 and it when from 0.06 to 0.003 which is a huge improvement!