Need Explanation of couchdb reduce function

From http://wiki.apache.org/couchdb/Introduction_to_CouchDB_views

The couchdb reduce function is defined as

function (key, values, rereduce) {
    return sum(values);
}

• key will be an array whose elements are arrays of the form [key,id]
• values will be an array of the values emitted for the respective elements in keys
• i.e. reduce([ [key1,id1], [key2,id2], [key3,id3] ], [value1,value2,value3], false)

I am having trouble understand开发者_开发百科ing when/why the array of keys would contain different key values. If the array of keys does contain different key values, how would I deal with it?

As an example, assume that my database contains movements between accounts of the form.

{"amount":100, "CreditAccount":"account_number", "DebitAccount":"account_number"}

I want a view that gives the balance of an account.

My map function does:

emit( doc.CreditAccount, doc.amount )
emit( doc.DebitAccount, -doc.amount )

My reduce function does:

return sum(values);

I seem to get the expected results, however I can't reconcile this with the possibility that my reduce function gets different key values.

Is my reduce function supposed to group key values first? What kind of result would I return in that case?

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By default, Futon "groups" your results, which means you get a fresh reduce per key—in your case, an account. The group feature is for exactly this situation.

Over the raw HTTP API, you will get one total reduce for all accounts which is probably not useful. So remember to use group=true in your own application to be sure you get summaries per account.