#29340: "cache.get_or_set()" extra hits on database
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Reporter: hematinik | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Core (Cache system) | Version: 2.0
Severity: Normal | Keywords: cache
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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I've found out that using {{{cache.get_or_set()}}} causes extra SQL
queries while using the alternative approach to get a key’s value or set a
value if the key isn’t in the cache (using "if" statement) works fine
without extra hits on database.
it seems the problem appears when you use the {{{cache.get_or_set()}}}
inside a loop . this is the code to reproduce the problem:
THE MODEL:
{{{
#!python
class EventManager(models.Manager):
def get_location(self,event):
output = cache.get_or_set(
'event_location_eID{}'.format(event.pk),
event.location,
None)
return output
class Event(models.Model):
objects = EventManager()
location = models.ForeignKey(Location,on_delete = models.CASCADE)
}}}
THE VIEW:
{{{
#!python
def home(request):
events = cache.get_or_set('all_events',Event.objects.all(),None) #
this line works fine without extra hits on db
locations = []
for event in events:
x = Event.objects.get_location(event) # this line causes extra SQL
queries while the cache keys already exist
locations.append(x)
return render(request,'index.html',{'locations':locations})
}}}
THE TEMPLATE:
{{{
#!python
{% for location in locations %}
{{location.title}} <br>
{% endfor %}
}}}
now if I change the {{{get_location()}}} method on {{{EventManager}}} to
this:
{{{
#!python
def get_location(self,event):
output = cache.get('event_location_eID{}'.format(event.pk))
if output is None:
output = event.location
cache.set('event_location_eID{}'.format(event.pk),output,None)
return output
}}}
there will be no more unnecessarily SQL queries.
I'm using **django-debug-toolbar** to monitor my database requests and the
information about queries is based on the statistics that **django-debug-
toolbar** provides. I'll also attach some screenshots from **django-
debug-toolbar** to clarify the situation.
Regards.
--
Ticket URL: <https://code.djangoproject.com/ticket/29340>
Django <https://code.djangoproject.com/>
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