On Mar 23, 9:55 pm, Javier Guerra Giraldez <jav...@guerrag.com> wrote:
> On Wed, Mar 23, 2011 at 8:49 PM, Brian Neal <bgn...@gmail.com> wrote:
> > items = Post.objects(filter=forum__topic__in=forums).order_by('-
> > topic__update_date', '-update_date').select_related(# as before)[:30]
>
> > But this had the same result as before. I checked the resulting SQL,
> > and it looked right, so I did an explain and it was pretty much as
> > before. It decided not to use the index.
>
> bummer :-(
>
> > So I broke it up into two queries:
>
> > topic_ids = list(Topic.objects.filter(forum__in=forums).order_by(
> > '-update_date').values_list('id', flat=True)[:30])
> > items = Post.objects.filter(topic__in=topic_ids).select_related(# as
> > before)[:30]
>
> did you try:
>
> items =
> Post.objects.filter(topic__in=Topic.objects.filter(forum__in=forums).order_
> by('-update_date')[:30]).select_related(#
> as before)[:30]
>
> IOW: use the first queryset for the '__in' clause, no need to flatten
> to a list of IDs. that allows the ORM to weave a subselect
> (seehttp://docs.djangoproject.com/en/1.3/ref/models/querysets/#in).
> unfortunately, the docs also warn about the less-than-ideal MySQL
> optimizer, so in many cases it turns to be faster to do just like you
> did. (i guess that specially in these limited-size queries).
I did try that. In fact, just remove the list() around the Topic query
from the code I posted above and that was what I tried first. I got a
DatabaseError with an elaborate message about how this type of nested
query or something wasn't supported by the version of MySQL I had.
Sorry I didn't save the exact error message.
Again, many thanks.
BN
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