[Tile-serving] [openstreetmap/osm2pgsql] Getting admin_centre node in process_relation (Discussion #1763)

[Martin Ždila]

Hello,

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

```lua
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?

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