#13906: REPEATABLE READ (as used by default on MySQL) breaks atleast
QuerySet.get_or_create().
-------------------------------------+-------------------------------------
Reporter: | Owner:
sebastian_noack | Status: new
Type: | Component: Database layer
Cleanup/optimization | (models, ORM)
Milestone: 1.4 | Severity: Normal
Version: SVN | Keywords: mysql transaction
Resolution: | isolation
Triage Stage: Accepted | Has patch: 1
Needs documentation: 1 | Needs tests: 0
Patch needs improvement: 1 | Easy pickings: 0
UI/UX: 0 |
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Comment (by akaariai):
SELECT FOR UPDATE does not solve this. At least in PostgreSQL if there is
no row there, then it does nothing. MySQL's documentation says it has the
same behavior. So, if there are 2 transactions inserting, then you would
have:
{{{
SELECT FOR UPDATE # Sees nothing, thus locks nothing.
# Another transaction inserts a row
# another transaction commits
INSERT # can't insert, IntegrityError
SELECT # can't see it.
}}}
I am not completely sure of this, but it seems that in REPEATABLE READ
there is nothing you can do. By definition you can't see the row of the
other transaction if it was not visible in the beginning of your
transaction. Unique index ensures you can't insert it.
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Ticket URL: <https://code.djangoproject.com/ticket/13906#comment:21>
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