#2465 - Make select_related() related work for ForeignKeys with null=True

[Rob Hudson]

http://code.djangoproject.com/ticket/2465

I'm running into this and it makes sense to me that select_related
should do this right. I'm curious of Adrian's comment about this
involving a bunch of LEFT JOINs and why that's a bad thing?

Thanks,
Rob

[Malcolm Tredinnick]

It's a lot slower at the SQL level. Try it and see: put two joined
tables into a database and allow the foreign key column to be NULL. Put
in 100,000 rows or so. Now select a joined version of the tables using
INNER joins and then LEFT OUTER joins. Time it. Notice the difference.
Imagine you were doing that a few times a second.

One thing we can add in the queryset-refactor, based on David Cramer's
work, is allowing specifying the fields to use in a select_related().
The default case will still work as now, but this allows more fine-tuned
control. In that case, if you specify a nullable column, we can join
across it because you will have explicitly said you are willing to bear
the pain (that wasn't something in David's original proposal, but it's a
logical extension, since the syntax remains the same). This will be
documented six ways from Sunday, which I fully expect people to not
read. That gives people who want to accept the performance trade-off the
ability to do so, without hurting the default case at all.

Malcolm

--
Works better when plugged in.
http://www.pointy-stick.com/blog/

[Rob Hudson]

On 9/15/07, Malcolm Tredinnick <mal...@pointy-stick.com> wrote:
>
> On Sun, 2007-09-16 at 00:36 +0000, Rob Hudson wrote:
> > http://code.djangoproject.com/ticket/2465
> >
> > I'm running into this and it makes sense to me that select_related
> > should do this right. I'm curious of Adrian's comment about this
> > involving a bunch of LEFT JOINs and why that's a bad thing?
>
> It's a lot slower at the SQL level. Try it and see: put two joined
> tables into a database and allow the foreign key column to be NULL. Put
> in 100,000 rows or so. Now select a joined version of the tables using
> INNER joins and then LEFT OUTER joins. Time it. Notice the difference.
> Imagine you were doing that a few times a second.

Thanks for the reply and explanation.

-Rob