#16649: Models.save() refactoring: check updated rows to determine action
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Reporter: akaariai | Owner: nobody
Type: | Status: new
Cleanup/optimization | Version: 1.3
Component: Database layer | Resolution:
(models, ORM) | Triage Stage: Design
Severity: Normal | decision needed
Keywords: | Needs documentation: 0
Has patch: 1 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 0
Easy pickings: 0 |
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Comment (by akaariai):
A bit more information about what the approach suggested is about. Assume
you load a model from DB, change some fields and save it. We currently do:
{{{
SELECT - load from DB
[change fields]; save()
In save:
SELECT - assert that the PK exists in the DB
if yes:
UPDATE
else:
INSERT
}}}
The second select above is usually redundant. If we know the object is
loaded from DB (self._state.adding == False), and we are saving it to the
same DB it was loaded from (self._state.db == the db we are saving to),
then it is very likely the object exists in the DB. So, instead we should
do:
{{{
In save:
if same db, not adding:
exists = UPDATE
else:
exists = SELECT
if exists:
UPDATE
if not exists:
INSERT
}}}
So, in the case the model is already in the DB, we do just a single update
instead of select + update. If it so happens it doesn't exist (which
should be very unlikely), we do update with zero rows modified and still
know to do an insert.
This does lead to behaviour change on MySQL when there is a concurrent
insert to the same table. Currently the SELECT sees nothing, and the
insert will lead to integrity error. After this, the update will block
until the inserting transaction is committed, and then update the matching
row. However, on PostgreSQL the update sees nothing, so there is no
behaviour change there. For MySQL users this will be better than what we
have now. For PostgreSQL users there is no change from behaviour
standpoint.
--
Ticket URL: <https://code.djangoproject.com/ticket/16649#comment:4>
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