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From Adam Kocoloski <kocol...@apache.org>
Subject Re: Frugal Erlang vs Resources Hungry CouchDB
Date Fri, 01 Jul 2011 15:57:00 GMT
On Jul 1, 2011, at 11:23 AM, Jens Alfke wrote:

> I am not sure whether the db looks up older revisions of documents by starting from an
earlier header (“going back in time”); I don’t think so, because this would be inefficient
(O(N)) for finding a specific revision of a document. Instead my hunch is that each document
points back to the position in the file of its previous revision. (Again, disclaimer, I am
extrapolating based on my knowledge of similar data structures.)

Hi Jens, in fact, each leaf in the ID btree contains stores a revision tree containing pointers
to _all_ available revisions of a document.  Retrieving an old revision (before compaction)
or a conflicting version of a document requires exactly the same number of IOs as retrieving
the current one.  Cheers,

Adam
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