How to disable Django query cache ?

Question

In my Django application, I repeatedly run the same query on my database (e.g. every 10 seconds). I then create an MD5 sum over the queryset I receive and compare that to the MD5 sum I created in the previous run. If both are equal, the data has not changed and the web page does not need updating.

While I do this, the data in the DB might change.

However, the query returns the same queryset, apparently due to query caching.

How can I disable the query cache and explicitely execute the query on the DB ?

Answer 1

How complicated are your QuerySet objects?

I think the simplest solution is to create a new QuerySet object and then evaluate it. That way you won't hit the QuerySet cache at all.

Answer 2

The link you provide to the Django Documentation implies that the following:

>>> print [e.headline for e in Entry.objects.all()]
>>> print [e.pub_date for e in Entry.objects.all()]

creates two queries to the database, whilst:

>>> queryset = Poll.objects.all()
>>> print [p.headline for p in queryset] # Evaluate the query set.
>>> print [p.pub_date for p in queryset] # Re-use the cache from the evaluation.

uses the query cache, as you are accessing the same evaluation results.

Answer 3

Query caching only applies within a QuerySet. In other words, if you evaluate the same queryset object twice, query caching will operate. But if you are doing a query every 10 seconds, presumably this is via a cron that spawns a new process each time, so there is no way Django will cache anything.

It is possible that your database's own cache will come into operation if you're repeatedly performing exactly the same query. You should look at the documentation for your DBMS to see how to manage that properly.

Answer 4

Thank you very much for your answers, your replies made me take a few steps back and rethink.

In order to test caching on a DBMS level, I went away from Django and used a shell script I anyway had handy to periodically query data from a SQLite db, while I added data in a second shell session. The new data showed up in the periodic queries right after I added them, so no query caching here.

This narrowed it down to the Django part. The code involved is not very complex and a little log output and a code review revealed the problem: The query used to obtain the queryset used in turn to create the MD5 sum had an error and was always empty. Therefore, the MD5 sum was always the same. Did indeed look like a cached result - data is changing, but the queryset stays the same. The problem did not show in the application, as a different query was used to obtain data displayed there.

Lesson learned: If you're completely puzzled, take a step back and re-think your assumptions.

Thanks again! :-)