Hi everybody.
I have come up with a way to create a tree where each node is a document
in CouchDB. The tree is ordered.
Maybe someone on the list tried something similar and could comment on
the approach. For others, this could inspire a new way of using CouchDB.
The performance features are:
* Fast reads.
* Slow writes (updates, deletions, moving subtrees).
The basic notion is that every document gets a path attribute that
specifies where in the tree the document is located.
The root path looks like this:
[0]
(JSON notation.)
A child of the root looks like this:
[0, 0]
Adding another child to the root provides this path:
[0, 32768]
(32768 equals 2**15 and it is somewhat arbitraryly chosen.)
The resulting documents could look like this:
{ '_id': 'root', 'path': [0] }
{ '_id': 'child_1', 'path': [0, 0] }
{ '_id': 'child_2', 'path': [0, 32768] }
Adding a child to child_2 yields the path [0, 32768, 0].
Now querying the whole tree is done using
startkey: [-2**16, 2**16]
endkey: [-2**16, 2**16, {}]
Querying the subtree rooted in child_1:
startkey: [0, 0]
endkey: [0, 0, {}]
I have based this on the collation rules found here:
http://wiki.apache.org/couchdb/View_collation#Complex_keys
The constants -2**16 and 2**16 are just endpoints where you sequentially
subdivide the interval into smaller segments to make room for more
siblings. Thus inserting a node between child_1 and child_2 will return
in the path [0, 32768/2] == [0, 16384]. As far as I understand it,
JavaScript uses 64 bits to store numbers (they are always floats), so
there should be ample room for many siblings under each node. At some
point, it may be needed to re-balance the tree, which will be an
operation requiring an update of most nodes in the subtree being balanced.
Any comments?
Regards,
Michael.
P. s. demonstration written in perl available on request.
