[Couchdb Wiki] Update of "Introduction to CouchDB views" by BrianCandler

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The following page has been changed by BrianCandler:
http://wiki.apache.org/couchdb/Introduction_to_CouchDB_views

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  See HttpViewApi to learn how to work with views. ["View_Snippets"] contain a few examples.
  
+ == Restrictions on map and reduce functions ===
+ 
+ The restriction on map functions is that they must be referentially transparent. That is, given the same input document, they will always emit the same key/value pairs. This allows CouchDB views to be updated incrementally, only reindexing the documents that have changed since the last index update.
+ 
+ To make incremental Map/Reduce possible, the Reduce function has the requirement that not only must it be referentially transparent, but it must also be commutative and associative for the array value input, to be able reduce on its own output and get the same answer, like this:
+ 
+ f(Key, Values) == f(Key, [ f(Key, Values) ] )
+ 
+ This requirement of reduce functions allows CouchDB to store off intermediated reductions directly into inner nodes of btree indexes, and the view index updates and retrievals will have logarithmic cost. It also allows the indexes to be spread across machines and reduced at query time with logarithmic cost.
+ 
+ For more details see [http://damienkatz.net/2008/02/incremental_map.html this blog post]
+