I haven't used the feature, but there is no reason to think that having the db
backend do a random sort is any slower than you doing it yourself.
If your query can reduce the number of rows returned to a relatively small
number, any "order by" should have acceptable performance. If not, it will be
expensive no matter who does the work.
hth
- Tom
On 2013-08-07, at 6:55 PM, Victor Hooi <victorh...@gmail.com> wrote:
> Hi,
>
> We have some code to return randomised Django model instances.
>
> Initially, I was using random.randint(), and returning objects by index
> between 0 < max_id.
>
> This was based on the Django docs:
>
> https://docs.djangoproject.com/en/dev/ref/models/querysets/#order-by
>
> specifically, the part:
>
> Note: order_by('?') queries may be expensive and slow, depending on the
> database backend you’re using.
>
> This blog post from February 2011 also seems to back that up:
>
> http://www.peterbe.com/plog/getting-random-rows-postgresql-django
>
> However, I just ran the benchmarks on my own box running Django 1.5 and
> Postgres 9.2.4, and the ORDER BY RANDOM() approach was slower, but not
> substantially so (certainly not to the extent hinted at by the blog post).
>
> I'm curious - what are other people's experiences with using ORDERY BY RANDOM
> versus other approaches?
>
> Has Postgres made some recent changes which makes ORDER BY RANDOM the better
> option now?
>
> Which database backends specifically should we be warned about when saying
> that order_by('?') is expensive and slow?
>
> Cheers,
> Victor
>
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