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From Apache Wiki <wikidi...@apache.org>
Subject [Couchdb Wiki] Update of "Introduction to CouchDB views" by RobertNewson
Date Sun, 30 Aug 2009 17:52:57 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Couchdb Wiki" for change notification.

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

The comment on the change is:
fix link.

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  [[Anchor(reduce_functions)]]
  === Reduce Functions ===
  
- Reduce is a powerful feature of CouchDB but is often misused which leads to performance
problems. From 0.10 onwards, CouchDB uses a heuristic to detect reduce functions that won't
scale to give the developer an early warning. A reduce function must reduce the input values
to a smaller output value. If you are building a composite return structure in your reduce,
or only transforming the values field, rather than summarizing it, you might be misusing this
feature. See [:#reduced_value_sizes] for more details.
+ Reduce is a powerful feature of CouchDB but is often misused which leads to performance
problems. From 0.10 onwards, CouchDB uses a heuristic to detect reduce functions that won't
scale to give the developer an early warning. A reduce function must reduce the input values
to a smaller output value. If you are building a composite return structure in your reduce,
or only transforming the values field, rather than summarizing it, you might be misusing this
feature. See [#reduced_value_sizes Reduced Value Sizes] for more details.
  
  If a view has a reduce function, it is used to produce aggregate results for that view.
A reduce function is passed a set of intermediate values and combines them to a single value.
Reduce functions must accept, as input, results emitted by its corresponding map function
'''as well as results returned by the reduce function itself'''. The latter case is referred
to as a ''rereduce''.
  

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