On Thu, Jan 29, 2009 at 10:08 AM, Frederik Ramm <frede...@remote.org> wrote:
> Hi, > > 80n wrote: > >> Can someone remind me again please. What's the purpose of calculating and >> storing the bbox for each changeset? >> > > So that it is possible to query the database effectively for something like > "all changesets possibly affecting London". Without changeset bboxes this > would mean retrieving all nodes in the London bbox, then retrieving all ways > that use these nodes, then retrieving all relations that use these nodes or > ways, then retrieve all changesets that affect any of these ways, nodes, or > relations, and then return them. This would be a very expensive operation - > and we expect *many* people to make these queries in regular intervals as > they're keeping a "watch" on their area! > > Of course the expensive method sketched above has the advantage of only > returning changesets that really affect the area in question, whereas the > bbox method we're using might also return a changeset that has one edit in > Southampton and one in Cambridge... but even if we want the precision of the > expensive method, the bbox method can be used as a kind of index. > Thanks, Frederik, that makes sense. I don't think I'd seen that use expressed anywhere up to now, but then I haven't really been paying that much attention to the 0.6 story. 80n > > Bye > Frederik >
_______________________________________________ dev mailing list dev@openstreetmap.org http://lists.openstreetmap.org/listinfo/dev
