#27193: ORDER BY clause not included in subqueries using select_for_update()
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Reporter: sqwishy | Owner: nobody
Type: Uncategorized | Status: new
Component: Uncategorized | Version: 1.9
Severity: Normal | Resolution:
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by sqwishy):
* status: closed => new
* resolution: needsinfo =>
Comment:
The documentation at the following link may suggest that rows are locked
while a query is running rather than all at once atomically; see
particularly the second paragraph under "13.2.1. Read Committed Isolation
Level" https://www.postgresql.org/docs/current/static/transaction-iso.html
#XACT-READ-COMMITTED
This is a bit of code where adding the order by seems to prevent
deadlocks.
{{{#!python
from django.db import transaction
from django.contrib.auth.models import Group
from django.db.models import F
def setup():
for i in range(20):
Group.objects.get_or_create(name='test_27193_%i' % i)
def test():
for _ in range(20):
with transaction.atomic():
Group.objects.filter(
id__in=Group.objects.filter(name__startswith='test_27193').select_for_update()
).update(name=F('name'))
#Group.objects.filter(
#
id__in=Group.objects.filter(name__startswith='test_27193').select_for_update().order_by('id')[1:]
#).update(name=F('name'))
setup()
test()
}}}
I have that saved to a file aptly named `script.py` and run it 20 times at
once (I'm sure there is a less ugly way to do this):
{{{
seq 20 | parallel --ungroup -j 20 "echo {}; ./manage.py shell --plain <
script.py"
}}}
Doing this with the script as shown, exceptions are raised in python about
deadlocks and PostgreSQL logs about it. If you comment out that first
query and uncomment the second one everything runs fine. The slice there
is just to convince django not to reset the order-by value in the
queryset.
--
Ticket URL: <https://code.djangoproject.com/ticket/27193#comment:2>
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