This is kind of already on the wiki under "How to store hierarchical
data". Reparenting massive amounts sucks, child creation doesn't. I
use this method. Though I store IDs in my path array instead of
integers. This makes it easy to find all items whose parent has ID x.
Or the section of the tree with IDs x, y, z.
On Nov 30, 2010, at 6:03 AM, Michael Zedeler <[email protected]> wrote:
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.
