Cast geography Polygon to MultiPolygon; function st_multi(geography) does not exist

Question

I'm trying to cast a geography Polygon to a MultiPolygon resulting from a 'geographic' intersection with a circle on Earth of a 10'000m radius, but PostGIS is raising this error when using ST_Multi on that resulting Polygon:

ERROR:  function st_multi(geography) does not exist
LINE 2:  ST_Multi(
         ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
SQL state: 42883
Character: 15

The message is not 100% clear to me, because ST_Multi does exist as I successfully used it on many other queries on that same database.

So, I have the feeling that it is simply not able to work on a 'geography' polygon (or maybe the error lies somewhere else?).

Here's a minimal reproducible code snippet (postgis 3.1 / postgres 13.1):

SELECT CAST(
  ST_Multi(
    ST_Intersection(
      ST_Simplify(
        ST_MakePolygon(
          ST_Force2D(
            ST_GeomFromGeoJSON(
              '{"type":"LineString",
              "coordinates":[
                  [8,48,45],
                  [8.1,47.2,45],
                  [8.2,47.3,45],
                  [8,48,45]
                ],
              "crs":{"type":"name","properties":{"name":"EPSG:4326"}}}'
            )
          )
        ),
        0.001
      ),
      ST_buffer(
        ST_SetSRID(
          CAST(ST_MakePoint(8.1, 47.2) AS GEOGRAPHY), '4326'),
        10000
      )
    )
  ) AS GEOMETRY
);

How would you cast a Polygon to a MultiPolygon in that particular case?

Answer 1

I figure out that apparently, ST_Multi does not appreciate to work on a geography feature.

Therefore, I actually have to use ST_Multi after casting the geography to a proper geometry feature (i.e. outside the cast function, and not inside).

This is working:

SELECT ST_Multi(
  CAST(
    ST_Intersection(
      ST_Simplify(
        ST_MakePolygon(
          ST_Force2D(
            ST_GeomFromGeoJSON(
              '{"type":"LineString",
              "coordinates":[
                  [8,48,45],
                  [8.1,47.2,45],
                  [8.2,47.3,45],
                  [8,48,45]
                ],
              "crs":{"type":"name","properties":{"name":"EPSG:4326"}}}'
            )
          )
        ),
        0.001
      ),
      ST_buffer(
        ST_SetSRID(
          CAST(ST_MakePoint(8.1, 47.2) AS GEOGRAPHY), '4326'),
        10000
      )
    ) AS GEOMETRY
  )
);

And if I encapsulate all this into ST_GeometryType it successfully tells:

st_geometrytype (text)
"ST_MultiPolygon"