#24096: GROUP BY on oracle fails with TextField and BinaryField
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Reporter: jarshwah | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by shaib):
* cc: shaib (added)
Comment:
I've just closed #20971 as a duplicate of this one, because the issue is
presented more clearly here.
Quoting my comment from that ticket:
> This seems to be a correctness problem on Oracle, but may cause
performance issues elsewhere (other backends will group over deferred
fields, in particular text-fields, which may indeed bring back the
problems of #17144).
Of jarshwah's options, I think option 4 (removing deferred columns from
the group-by) is a no-brainer (even if the PK is not available). I find
option 3 unacceptable (except on MySql, perhaps... no, I'm kidding), and
option 1 vastly inferior.
With respect to removal, I don't like the idea of completely automatic
removal. Too much magic for my tastes. But -- having actually implemented
such automatic removal for a project I was involved with (in a custom
query-set class) -- I do see the need for it to be easy; and per my
comment above, I also think we should tell non-Oracle users about it.
So my suggestion is:
1. Add a database-feature `can_compare_LOBs`
2. When a query groups over a LOB -- if `can_compare_LOBs`, warn about
performance implications; else, error out (with a clear Django exception
rather than an obscure Oracle error)
3. Add a query-set method to defer all LOBs, and/or a query-set subclass
that does it automatically.
--
Ticket URL: <https://code.djangoproject.com/ticket/24096#comment:4>
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