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Subject: Re: averaging and rereduce
From: Jan Lehnardt <jan@apache.org>
In-Reply-To: <7FBFD14C-8236-4317-A868-B3DC1F8C2EEB@freshout.us>
Date: Sun, 30 Aug 2009 12:27:12 +0200
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References: <7FBFD14C-8236-4317-A868-B3DC1F8C2EEB@freshout.us>
To: user@couchdb.apache.org


On 30 Aug 2009, at 11:36, Francisco Viramontes wrote:

> Hi list!
>
> I hava a question about grouping and the reduce function
>
> If aI want to average the sum of manu values returned by my map  
> function on a large database (~100000 rows ) the reduce function  
> gets called with the rereduce boolean
>
> The thing is a am not sure if its mathematically correct to average  
> sets of floats in groups as optimized by couch where I can have to  
> average smaller sets of previously averaged values. I certainly dont  
> get the exact same result of averaging a whole set than a grouped set.
>
> My question is how can I handle my reduce function to reduce a whole  
> set and not in groups?
>
> Note the emit call is done with complex keys:
>
> map:
>
> function(doc) {
>       if ((doc['couchrest-type'] == 'RawData') && doc 
> ['device_variable_id']) {
>         date =  new Date(doc.date_time);
>         emit([doc.device_variable_id, date.getFullYear(),  
> date.getUTCMonth() + 1, date.getUTCDate(), date.getUTCHours(),  
> parseInt(date.getUTCMinutes() / 20 + 1) ], doc.value);
>       }
>     }
>
> reduce:
>
> function( keys, values, rereduce ) {
>     if(rereduce){
>               log("rereduce");
>             }
>       return sum( values ) / values.length ;
>     }

You cannot ignore the call where rereduce is true. See http://svn.apache.org/viewvc/couchdb/trunk/share/www/script/test/reduce.js?revision=804727&content-type=text%2Fplain

scroll down a little:

"  var reduceCombine = function (keys, values, rereduce) { "

for an example.