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The following page has been changed by BrianCandler: http://wiki.apache.org/couchdb/Introduction_to_CouchDB_views The comment on the change is: Add some links and a stronger restriction on reduce/rereduce processing ------------------------------------------------------------------------------ For more details see [http://damienkatz.net/2008/02/incremental_map.html this blog post] + Furthermore: you have no control over how documents are partitioned in reduce and re-reduce phases. You cannot rely on "adjacent" documents all being presented at the same time to the reduce function; it's quite possible that one subset will be reduced to R1, another subset will be reduced to R2, and then R1 and R2 will be re-reduced together to make the final reduce value. In the limiting case, consider that each of your documents might be reduced individually and that the reduce outputs will be re-reduced together (which also may happen as a single stage or in multiple stages) + + So you should also design your reduce/re-reduce function so that + + {{{ + f(Key, Values) == f(Key, [ f(Key, Value0), f(Key, Value1), f(Key, Value2), ... ] ) + }}} + === Reduced Value Sizes === As CouchDB computes view indexes it also calculates the corresponding reduce values and caches this value inside each of the btree node pointers. This scheme allows CouchDB to reuse reduced values when updating the btree. This scheme requires some care to be taken with the amount of data returned from reduce functions. @@ -225, +233 @@ See [http://labs.mudynamics.com/2009/04/03/interactive-couchdb/ this blog posting] for an interactive tutorial (emulator in JavaScript) that explains map/reduce, view collation and how to query CouchDB RESTfully. + == Implementation == + + These blog posts include information about how map/reduce works, including how reduce values are kept in btree nodes. + * [http://damienkatz.net/2008/02/incremental_map.html] + * [http://damienkatz.net/2008/02/incremental_map_1.html] + * [http://horicky.blogspot.com/2008/10/couchdb-implementation.html] + + These mailing list posts may also be helpful: + * [http://mail-archives.apache.org/mod_mbox/couchdb-user/200903.mbox/<20090330084727.ga7...@uk.tiscali.com>] + * [http://mail-archives.apache.org/mod_mbox/couchdb-user/200906.mbox/<20090621202105.ga1...@uk.tiscali.com>] +
