#30375: Use "NO KEY" when doing select_for_update for PostgreSQL
-------------------------------------+-------------------------------------
Reporter: mudetz | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database | Version: 1.11
layer (models, ORM) | Keywords: postgres, lock,
Severity: Normal | database, operation
Triage Stage: | Has patch: 1
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Currently Django compiles select for update into one of
- FOR UPDATE
- FOR UPDATE NOWAIT
- FOR UPDATE SKIP LOCKED
All of these acquire a lock which conflicts with `KEY SHARE` locks. This
means that updates on tables which references a row locked by `FOR UPDATE`
will have to wait until the lock is released. This is to avoid any
unexpected changes on `PRIMARY KEY` fields but, **in Django, primary key
fields are read-only, so locking them makes no sense**.
There is no need to acquire these kind of locks, instead, we should (at
least be able to) use
- FOR NO KEY UPDATE
- FOR NO KEY UPDATE NOWAIT
- FOR NO KEY UPDATE SKIP LOCKED
which can be easily done by overriding BaseOperation.for_update_sql in
Postgres DatabaseOperation
{{{
def for_update_sql(self, nowait=False, skip_locked=False):
"""
Returns the FOR UPDATE SQL clause to lock rows for an update
operation.
"""
if nowait:
return 'FOR NO KEY UPDATE NOWAIT'
elif skip_locked:
return 'FOR NO KEY UPDATE SKIP LOCKED'
else:
return 'FOR NO KEY UPDATE'
}}}
Not doing so causes lock contention in our use case.
--
Ticket URL: <https://code.djangoproject.com/ticket/30375>
Django <https://code.djangoproject.com/>
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