Getting admin_centre node in process_relation

[zdila]

Hello,

I need node with name and geometry (point) in process_relation function. My current approach is following:

local node_table = {};

function osm2pgsql.process_node(object)
  if object.tags.place then -- consider only places not to fill-up whole RAM
    node_table[object.id] = { point = object:as_point(), name = object.tags.name }
  end
end

function osm2pgsql.process_relation(object)
  local type = object.tags.type;

  for _, member in ipairs(object.members) do
    if member.role == 'admin_centre' and member.type == 'n' then
      local node = node_table[member.ref]

      // now I can use `node.name` and `node.point`
    end
  end
end

Is it correct? I mean it works but it requires storing potential nodes to node_table which eats RAM. I would expect to be able to access nodes (and ways) including geometry in process_relation via some API but I couldn't find it.

Maybe another approach could be to store references to admin_centre nodes in select_relation_members and then look them up in process_node to save some memory?

[joto]

You can get a collection of all member geometries of a relation with object:as_geometrycollection() and then get the nth geometry with geometry_n(). So something like this should work:

function osm2pgsql.process_relation(object)
  local geom = object.as_geometrycollection()
  for i, member in ipairs(object.members) do
    if member.role == 'admin_centre' and member.type == 'n' then
      local point_geom = geom.geometry_n(i)
      ... 
    end
  end
end

But that only gives you the geometry, not the name.

If you don't want to store all place points, but only those actually mentioned in a relation, you can look at two-stage processing. I's a bit tricky to get right though.

You can also just take advantage of the database you have. Add those place nodes to a database table, create another table from the relation members and join them afterwards.

[zdila]

Thanks @joto for your valuable hints!