On 11/07/2009, Devel63 <danstic...@gmail.com> wrote:
>
> Barry, I understand your objections below, but do you have a better
> approach?
no, I dont. The way in my first reply is the best I can think off. Not
totally uniform, but dont think any 'random' solution is going to be,
unless you keep the data uptodate - which is not practical on
AppEngine.
A reasonably good solution, is to use a FIFO buffer, when the queue is
empty you get all keys, and shuffle them and add to the queue. then
getting random results is a matter of poping that many keys off the
queue, and fetching them.
Dont know quite how would implement a fifo buffer on AppEngine, would
almost certainly use the datastore, so is going to be expensive.
>
> Assigning random numbers to entities is guaranteed to be worse. If
> you are worried about an entity being deleted and opening a gap in the
> sequence, imagine the thousand-fold gaps you will see with random ID
> generation (e.g. 1, 10001, 10002, 20000, ...).
>
> See below.
>
> On Jul 10, 3:03 pm, Barry Hunter <barrybhun...@googlemail.com> wrote:
>
> > On 10/07/2009, Devel63 <danstic...@gmail.com> wrote:
> >
> >
> >
> > > The best way is to assign a one-up counter to each record as you
> > > create it, then call random.randint(1,max_counter) to determine the
> > > desired record.
> >
> > > To retrieve multiple random entities in a query, do a filter('IN ',
> > > [my random nums]).
> >
> > doent work that well once records start getting deleted (get the same
> > issue, non uniform distribution)
>
>
> If an app needs to support entity deletion, you can still ensure
> uniformity by running a periodic cron job to compress the counter
> sequence.
>
>
> >
> > nor does it work if you filtering at the same time :(
> >
>
>
> Correct, in that the distribution is no longer uniform. But this is
> also true of the random ID approach. I admit that the random ID
> approach seems appealing at first, but when you actually look into it,
> you'll find that you are guaranteed that many results will be 3X more
> likely than others, or worse. It IS better in the case that you want
> to randomize based on time of entity creation, but there are other
> ways to deal with this.
>
> I would love it if someone could come up with a good way to do true
> random results of an arbitrary query set!!
>
>
> >
> >
> > > Note that behind the scenes this generates multiple queries, so you're
> > > not saving much time.
> >
> > > On Jul 10, 7:34 am, Wooble <geoffsp...@gmail.com> wrote:
> > > > Highly non-optimal solution: have a cron job assign new random numbers
> > > > to your entities often enough to simulate randomness. Even just re-
> > > > assigning numbers to entities that have been previously selected might
> > > > work. This involves a lot more CPU as you'd be doing writes, but
> > > > shifts the work from request time to a background process so your
> > > > users don't see the added latency for doing N queries.
> >
> > > > Another possible solution would be to fetch keys only for X*N entities
> > > > (where greater X's produce more apparent randomness) then choose N of
> > > > those keys to actually fetch entities.
> >
> > > > On Jul 9, 12:33 pm, aloo <aleem.maw...@gmail.com> wrote:
> >
> > > > > Hi all,
> >
> > > > > I'm trying to write a GQL query that returns N random records of a
> > > > > specific kind. My current implementation works but requires N calls
> to
> > > > > the datastore. I'd like to make it 1 call to the datastore if
> > > > > possible.
> >
> > > > > I currently assign a random number to every kind that I put into the
> > > > > datastore. When I query for a random record I generate another
> random
> > > > > number and query for records > rand ORDER BY asc LIMIT 1.
> >
> > > > > This works, however, it only returns 1 record so I need to do N
> > > > > queries. Any ideas on how to make this one query? Thanks.
> >
> >
> >
>
--
Barry
- www.nearby.org.uk - www.geograph.org.uk -
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