[Tile-serving] [osm2pgsql-dev/osm2pgsql] How to filter (postgis) Multipolygon in LUA (Discussion #2357)

[Tobias]

**Some context:**

We are doing some work of our processing in LUA and some in SQL.
We are only interested in areas data for this specific part of the code. But since OSM has this setup where areas with a hole are relations, we have to ways and relations.
With relations, we now get multiple data types
- relations which are in fact a Multipolygon (separate areas)
- relations which are a Polygon with a hole

We want to process the Polygons and insert them in a table with geometry type Polygon.
And as such skip the Multipolygon areas.

(The data we are working with are small areas (parking spaces), not large relations, so osm2pgsqls auto-splitting ([ref](https://osm2pgsql.org/doc/manual-v1.html#single-vs-multi-geometries)) is no concern here.)

**What is the best way to do this?**

- We cannot use `object.type` because that is just `relation` https://osm2pgsql.org/doc/manual-v1.html#processing-callbacks
- We call `object:as_multipoint()` and hope it returns `nil` if that is not possible, because it explodes.
- We could start try-catching in LUA but that does not feel right https://www.lua.org/pil/8.4.html

**Our current approach:**

Code: https://github.com/FixMyBerlin/tilda-geo/blob/develop/processing/topics/parking/separate_parkings/parking_separate_parking_areas.lua

1. We set our table definition to `type = 'polygon'`
2. We then get our geometry with `geom = object:as_multipolygon()`
3. We then check if `geom:num_geometries() == 1` and only then insert the data
4. (We log the other case for debugging and cleanup)

I this the way you would solve this?

A different approach might be to check the `.members` and their `role` and see if more than one outer ring is present. (https://osm2pgsql.org/doc/manual-v1.html#single-vs-multi-geometries) But we did not try that since the above seemed more straight forward.

**Update the docs?**

If this is the recommended approach, would this be something for the docs?
Eg in https://osm2pgsql.org/doc/manual-v1.html#single-vs-multi-geometries we could add a paragraph like…

> The best way to detect if a relation will likely be transformed into a Polygon or MultiPolygon is by checking `num_geometries`. `object:as_multipolygon():num_geometries() == 1` are Polygons, > 1 are MultiPolygons.

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