This page that you linked explained it.

The view (which is the whole point of map reduce in CouchDB) can be updated by re-indexing only the documents that have changed since the last index update. That's the incremental part.

This can be achieved by requiring the reduce function to be referentially transparent, which means that it always returns the same output for a given input.

The reduce function also must be commutative and associative for the array value input, which means that if you run the reducer on the output of that same reducer, you will receive the same result. In that wiki page it is expressed like:

f(Key, Values) == f(Key, [ f(Key, Values) ] )


Rereduce is where you take the output from several reducer calls and run that through the reducer again. This sometimes is required because CouchDB sends stuff through the reducer in batches, so sometimes not all keys that need to be reduced will be sent through in once shot.

Why is MapReduce in CouchDB called "incremental"?
I know that the OS will sometimes initialise memory with certain patterns such as 0xCD and 0xDD.  What I want to know is when and why this happens.

When

Is this specific to the compiler used?

Do malloc/new and free/delete work in the same way with regard to this?

Is it platform specific?

Will it occur on other operating systems, such as Linux or VxWorks?

Why

My understanding is this only occurs in Win32 debug configuration, and it is used to detect memory overruns and to help the compiler catch exceptions.

Can you give any practical examples as to how this initialisation is useful?

I remember reading something (maybe in Code Complete 2) that it is good to initialise memory to a known pattern when allocating it, and certain patterns will trigger interrupts in Win32 which will result in exceptions showing in the debugger.

How portable is this?
