#35399: Reduce the "Case-When" sequence for a bulk_update when the values for a
certain field are the same.
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Reporter: Willem Van Onsem | Owner: nobody
Type: | Status: closed
Cleanup/optimization |
Component: Database layer | Version: 5.0
(models, ORM) |
Severity: Normal | Resolution: duplicate
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 Willem Van Onsem):
Hi, that is already in the changeset: in case the item is not hashable, it
will raise a type error: we catch that, and then fallback on the original
way of doing it, without much delay. So it is more a boosting mechanism
that when fails, is "disabled" when running through it.
The hash of an expression indeed is take more time than the one of an
`int`, but the idea is that this often will still pay off, since the
cascade of Case`-`When` essentially forces the database into linear
search, while it *might* be possible that a certain database finds a way,
if that is not the case, it will first look at the first `When`, if that
fails, the next one, etc. so for ~1000 `When`s, it will on average take
500 items per row.
I agree that a temporary table is probably more ideal.
--
Ticket URL: <https://code.djangoproject.com/ticket/35399#comment:6>
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