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2018 January 20
DP
кто знает как лучше воду из полигонов удалять на лету? https://gis.stackexchange.com/questions/268623/best-way-to-remove-water-from-osm-polygons
нужно иметь в виду strictness у ST_Difference:
Y
Спасибо, а во что нужно coalesce ST_UNION() сейчас?
DP
можно вообще ST_Difference в coalesce
Y
можно вообще ST_Difference в coalesce
Not sure what you mean. If ST_UNION() -> null, wouldn't ST_difference(geo, null) produce null instead of geo?
DP
да, и coalesce(ST_difference(geo, null), geom) вернёт geom
Y
@Komzpa thanks. What about efficiency? In my query, what is the best stage at which to do the diff? https://gis.stackexchange.com/questions/268623/best-way-to-remove-water-from-osm-polygons
DP
depends on your data structure and scatter of polygons per wikidata id
DP
you probaly want to take osm polygons, for each of them take a union of water that intersects it, subtract water from each of them and then union all the polygons that are left
DP
зачем там ST_Multi?