I am reading the O'Reilly CouchDB book. I am puzzled by the reduce/re-reduce/incremental-MapReduce part on page 64. Too much is left to rhetory in the O'Reilly book with the sentence


  If you're interested in pushing the ede of CouchDB's incremental reduce functionality, have a look at Google's paper on Sawzall, ...


If I understand the word "incremental" correctly, it refers to some sort of addition -operation in the B-tree data structure. I cannot yet see why it is somehow special over typical map-reduce, probably not yet understanding it. In CouchDB, it mentions that there is no side-effects with map function - does that hold true with reduce too?

Why is MapReduce in CouchDB is called "incremental"?

Helper questions


Explain the quote about incremental MapReduce with Sawzall.
Why two terms for the same thing i.e. reduction? Reduce and re-reduce?


References


A Google paper about Sawzall.
Introduction to CouchDB views in the CouchDB wiki and a lot of blurry blog references.
CouchDB O'Reilly book



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