[OSM-dev] Minute Diffs Broken

Brett Henderson brett at bretth.com
Wed May 6 00:26:38 BST 2009

Greg Troxel wrote:
> Brett Henderson <brett at bretth.com> writes:
> Given the use of pgsql transactions, osmosis won't see data from
> uncommitted transactions.  So I really meant "changes in the database,
> subject to the notion that uncommitted transactions won't be visible."
Hehe, terminology strikes again.  I think I know what you mean :-)
>> To complicate things slightly further, the full history files
>> http://planet.openstreetmap.org/history/
>> are similar but complete a full delta from one point in time to
>> another and may contain several versions of a single entity.
>> So perhaps the term "diffs" is the right one for the existing files
>> and "deltas" is the right one for full history files.
> I would hope that both have the property that if a copy of the DB that
> was right at the earlier time, then applying delta or diff to that copy
> gets one a copy of the database as of when the osmosis extract
> transaction ran.  Perhaps then the delta has the intermediate steps and
> the diff is permitted to collapse them?
That's correct and exactly how it works.  Both deltas and diffs (if I 
can call them that) extract the full history between two points in time, 
but the diffs collapse the changes into a minimal set.  Diffs are fine 
if you just want the latest snapshot, deltas are required to replicate 
full history.
>> The reason I've tended to avoid the word "diffs" is because the planet
>> directory also contains diffs between planet files.  These diffs are
>> yet another way of describing changes/differences and are truly a
>> difference between two planet files.
> As in the output of the diff command on two text files which happen to
> contain xml, it sounds like.
It's not the same as the diff command either.  It produces output like:

    <node id="80871" lat="58.4206038728957" lon="15.5648050309828" 
    <node id="80871" lat="58.4206038728957" lon="15.5648050309828" 

which is very similar to the osmosis diffs but it treats a modify as a 
delete and an add.  Osmosis just creates a modify element containing the 
new version and leaves out the old version.
>> If you're not familiar with it already, please check out the API
>> schema.  If information isn't stored there, we can't query it.  For
>> example, there is no concept of an upload in the database, the only
>> grouping feature it has is changesets.
>> http://gweb.bretth.com/apidb06-pgsql-latest.sql
> Thanks - read and sort of understood - there's a lot in there.
 From memory the only tables that really matter are the nodes, 
node_tags, ways, way_tags, way_nodes, relations, relation_tags, 
relation_members, changesets, and users.  The current_* tables are 
irrelevant.  And the rest I don't understand at all ;-)

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