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From "Jeff Hinrichs - DM&T" <dunde...@gmail.com>
Subject Re: reduce/rereduce confusion
Date Wed, 21 Jan 2009 02:50:24 GMT
On Tue, Jan 20, 2009 at 7:47 PM, Adam Wolff <awolff@gmail.com> wrote:
> Hi everyone,I'm really excited about CouchDB and I've started playing with
> it. I get all of it, except for reduce, and especially re-reduce.
>
> My first question is: how does CouchDB maintain all the separate output for
> a given key from the map function? I mean: given a simple reduce that just
> sums results, how does couch maintain separate results for each possible
> key/key range that can be given as input to that view?
>
> My second question: when and why does rereduce get called? Is this simply to
> allow the server to chunk the processing, or is there semantic meaning to
> it? I had assumed the former -- it's just a way of limiting the size of the
> input to the reduce function -- but then this really confused me: if I log
> each time my reduce function gets called, I see that the last time it's
> called, it's with rereduce=false. How is this possible? Don't all the
> results have to be funneled through rereduce to produce a single result
> value?
>
> Any help here would be much appreciated. If there's a resource on the web I
> should look at, please send it my way. Thanks!
>
> A
Being that I just went through the learning process on reduce, I'll
point you here:
http://wiki.apache.org/couchdb/Introduction_to_CouchDB_views
"Reduce Functions"

As a good place to start.
Also, the mailing list, is an excellent resource.
http://mail-archives.apache.org/mod_mbox/couchdb-user/200901.mbox/%3c61B374C7-34D7-45C3-9F8B-F11EFD77303D@apache.org%3e

along with:
http://en.wikipedia.org/wiki/MapReduce
http://labs.google.com/papers/mapreduce.html
and
http://damienkatz.net/2008/02/incremental_map.html

Regards,

Jeff

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