On Fri, Jun 12, 2009 at 10:25 AM, William Temperley <[email protected] > wrote:
> > Thanks - it's making some sense now, though for me this seems somewhat > confusing behaviour. > I'd been expecting behaviour similar to shp2pgsql where you can > specify arbitrary queries, which don't get messed with. Mapnik > rewrites the query in somewhat non-transparent ways. Yeah, it's something I've been thinking about cleaning up. Currently by default it: - find the geometry from the geometry_columns table - figures out whether it needs to do transforms based on the map projection & the geometry field projection - takes all your filters and figures out which attribute fields it needs - constructs a query for those attribute fields and the geometry - does a where clause for the map extent against the geometry field - runs it Bad things: - filters are all done in mapnik (if you're drawing all the data that's fine, otherwise it gets too much data out) - if you would rather reproject in postgis it gets messy - if you have multiple geometry fields you need to use my geometry_field= thing - using custom sql is ... interesting - the general pattern is table="(SELECT blah FROM ... WHERE ...) AS x", but you need to make sure field names all line up. Good things: - doesn't pull unnecessary attribute fields from the database (ala SELECT * FROM) - edit the map xml / script, add a new filter and it just works - great for simple use cases > I've ended up hacking postgis.cpp, allowing the table parameter to > pass straight through and be executed unadulterated (i.e. I can pass > arbitrary queries shp2pgsql style, provided I use "AsBinary(the_geom) > as geom" to get the geometry. Works well for me. Can provide a patch > if anyone wants. I've been wondering whether we do something like: table= -> current behaviour sql= -> all on your own, you need to make sure the fields/geometries/everything are correct. Opinions? Rob :) -- Koordinates Ltd. PO Box 1604, Shortland St, Auckland 1140, New Zealand Phone +64-9-966 0433 Fax +64-9-969 0045 Web http://koordinates.com/
_______________________________________________ Mapnik-users mailing list [email protected] https://lists.berlios.de/mailman/listinfo/mapnik-users
