#35399: Reduce the "Case-When" sequence for a bulk_update when the values for a
certain field are the same.
-------------------------------------+-------------------------------------
Reporter: Willem Van Onsem | Owner: nobody
Type: New feature | Status: new
Component: Database layer | Version: 5.0
(models, ORM) |
Severity: Normal | Resolution:
Keywords: db, bulk_update, | Triage Stage:
case, when | Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Simon Charette):
Thank you for taking the time to perform the benchmarks, I think you
missed the point brought up about hashing being expensive on non-literal
values though.
In other words `int.__hash__` is pretty fast but `Expression.__hash__`
isn't and you need to perform an implicit one to gather values in
`defaultdict(list)`. Since `bulk_update` support both expressions and
literal assignment the benchmark must be run against both to be
representative.
Try running them with assignments of `F("scans_in") +
Value(random.randint(0, m))` instead and you should see a slowdown.
The need for _hashability_ also poses another problem. What should be done
with values that are not hashable such as `JSONField` assignment of
`dict`s?
{{{#!python
objects = [
Object(pk=1, data={"foo": "bar"}),
Object(pk=2, data={"foo": "bar"}),
]
Object.objects.bulk_update(es, fields=["data"])
}}}
When all of these edge cases are accounted for (we'll need a route for
non-hashable values mixed with hashable values) I still believe that time
would be better spend not using `CASE` / `WHEN` at all and favor investing
efforts towards #29771 and #31202.
--
Ticket URL: <https://code.djangoproject.com/ticket/35399#comment:4>
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