I apologize in advance as this is probably only tangentially related
to Django.
I have a forums type Django application. In my RSS feed class I had
this Django model query:
return Post.objects.filter(topic__forum__id=obj.id).order_by(
'-creation_date').select_related(depth=2)[:30]
It seemed to work okay, but was slow (2 seconds). I was able to reduce
the time it took by more carefully extracting only the fields I needed
with values_list().
But then I changed it to use an "in" operation:
forums = [1, 2, 3, 4, 5]
return Post.objects.filter(topic__forum__in=forums).order_by(
'-creation_date').select_related(depth=2)[:30]
This query took over 40 seconds! Again, I can reduce this time using
values_list and a smaller list of fields, but even then it is 4
seconds or so.
I did an EXPLAIN on each of the queries, see the link below. The top
one is the fast one, and the bottom one is the slow one:
http://dpaste.com/524697/
I am not a MySQL expert and I need help interpreting the results. It
looks like for whatever reason, on the slow one, MySQL identified a
possible index, but then decided not to use it?
I ended up working around it by doing the work in Python: I looped
over all the forum ids in the "in" list, making the first query for
each, and then had to combine and sort the results myself.
I'm using InnoDB if that matters. Did I do something wrong or is this
a MySQL issue?
Thanks for any insights.
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