[OSM-dev] Daily Diff parsing - tile expiring

Brett Henderson brett at bretth.com
Thu Aug 21 00:52:12 BST 2008

This sounds much simpler :-)

Frederik Ramm wrote:
> Hi,
>> I think that adding the Lat/Lon tag to node references in the ways is not an 
>> option since it would require a huge amount of queries on the database when 
>> creating the diffs. The positive side of this would that it would only have 
>> to be done once.
> If you are happy with supporting only zoom level 16 granularity, you can 
> store the tile x and tile y coordinates for each node, that's only 4 
> bytes per node.
> If you create a file that contains four bytes for every node, indexed 
> simply by node id (node #1 x at file position 0, node #1 y at 2, node #2 
> x at 4 etc.), then your file will have about 1.2 Gig for the current 
> planet. When processing a diff you would simply mmap that file, 
> effectively loading into memory and granting very fast access to node 
> positions.
> This wastes some memory because it reserves space for deleted nodes as 
> well, but when I last looked the number of "deleted" node ids was about 
> 20%, and this is an acceptable overhead - otherwise you would have to 
> use something like a hashtable to index your node list which would use 
> more than that.
> Bye
> Frederik

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