[OSM-dev] osm2pgsql tile expiry freaks me out

[Steve Hill]

On Thu, 5 Nov 2009, Jon Burgess wrote:

> I have discussed the expiry scripts with Matt a couple of times and he
> found that the osm2pgsql based approach tended to hit the DB quite hard.

Unfortunately there isn't really a way around hitting the database - 
ISTR that where possible, the expiry code tries to use data it already has 
in memory, but for some data (such as the deletion of an old object) there 
just isn't anything you can do other than going to the database to find 
out where that object was.

> He found that even though the osm2pgsql code should in theory produce
> more accurate results, the ruby scripts tended to work better overall.

If the Ruby scripts produce more accurate results, we should certainly 
look at why and try to improve the osm2pgsql code.  The ruby scripts 
probably don't expire as many tiles - I think they only expire metatiles 
that have a node associated with a changed object on them, which means 
that it won't catch cases where a way crosses the corner of a metatile 
(with no nodes on the metatile) and where a filled polygon changes 
(resulting in a need to re-render all the tiles within it).

-- 

  - Steve
    xmpp:steve at nexusuk.org   sip:steve at nexusuk.org   http://www.nexusuk.org/

      Servatis a periculum, servatis a maleficum - Whisper, Evanescence