#30375: Use "NO KEY" when doing select_for_update for PostgreSQL
-------------------------------------+-------------------------------------
Reporter: Manuel Weitzman | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: postgres, lock, | Triage Stage:
database, operation | 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 Manuel Weitzman):
Replying to [comment:2 Simon Charette]:
> > ... in Django, primary key fields are read-only, so locking them makes
no sense
>
> That's not true AFAIK. Even if `AutoField` are backed by sequences and
generated on the database side they can still be overridden by developers.
Same thing with non-`AutoField` primary keys (e.g.
`UUIDField(default=uuid.uuid4)`).
Then the docs are wrong or at least misleading: ''"The primary key field
is read-only. If you change the value of the primary key on an existing
object and then save it, a new object will be created alongside the old
one."" (https://docs.djangoproject.com/en/2.2/topics/db/models/)
Which means that UPDATEs over PKs are transformed into INSERTs
> On the other hand updating a primary key while holding a lock is really
uncommon so I wouldn't be against adding a `select_for_udate(primary_key)`
parameter that defaults to `False`. We just have to make sure that we
still allow these operations to be performed somehow.
That is possible, I tested today and only a few changes are required to
achieve this behavior. Maybe I could send a PR soon for review?
--
Ticket URL: <https://code.djangoproject.com/ticket/30375#comment:3>
Django <https://code.djangoproject.com/>
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