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Date: Tue, 21 Sep 2010 22:49:19 +0100
Message-ID: <AANLkTi=K8=k14wmmAVjb=NQX+oagH-38TRcZ=YqUcB71@mail.gmail.com>
Subject: Re: Random Document
From: Peter Braden <PeterBraden@PeterBraden.co.uk>
To: user@couchdb.apache.org
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Hi,

I'm after a) - the equivalent of a 'SORT BY RANDOM LIMIT x' sql statement.

> But as this isn't deterministic, I'm pretty sure it's wrong.
> I don't follow your logic. The view will show all documents in a random
order. The fact that is is unrepeatable may make it useless for > your
purposes, but it does not make the maths invalid, or the statistics wrong.

As far as I know, the couchdb internals rely on the fact that view keys are
deterministic to do their view updates.

I'm not entirely convinced that my current function produces a good random
selection - if a document is updated more, and therefore it's view entry is
updated more, does that mean it has a different chance of being selected?

Cheers,

Peter


On 21 September 2010 20:25, Ian Hobson <ian@ianhobson.co.uk> wrote:

> On 21/09/2010 18:27, Peter Braden wrote:
>
>> Hi,
>>
>> Is there a good way to get a random document from a database.
>>
> Hmm, that depends upon what you mean by "good", and "random" and if you
> want a repeatable result! I guess I'm asking what exactly are you trying to
> do?
>
> a) Pick a representative, and statistically defensible sample of size X
> from a population of Y documents where each document has an equal
> probability of being selected, and cannot be selected twice.
>
> b) Take a sample of size 1 from a population of Y, X times (so a given
> document could be taken more than once)?
>
> c) Something similar to a or b where you don't know Y in advance?
>
> d) Shuffle the documents?
>
>
>  I'm currently
>
>> using a view that does:
>>
>> function(doc) {
>>     emit(Math.random(), doc);
>> };
>>
>> But as this isn't deterministic, I'm pretty sure it's wrong.
>>
> I don't follow your logic. The view will show all documents in a random
> order. The fact that is is unrepeatable may make it useless for your
> purposes, but it does not make the maths invalid, or the statistics wrong.
>
> Regards
>
> Ian
>


-- 
--
Peter Braden

<http://PeterBraden.co.uk/>

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