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Date: Tue, 20 Jan 2009 17:47:16 -0800
Message-ID: <e8d26ac40901201747jdb7ed17w7b19fc73a55320b1@mail.gmail.com>
Subject: reduce/rereduce confusion
From: Adam Wolff <awolff@gmail.com>
To: couchdb-user@incubator.apache.org
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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

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