-- Function by Stuart Lynn for a simple interpolation of a value
-- from a polygon table over an arbitrary polygon
-- (weighted by the area proportion overlapped)
-- Aereal weighting is a very simple form of aereal interpolation.
--
-- Parameters:
--   * geom a Polygon geometry which defines the area where a value will be
--     estimated as the area-weighted sum of a given table/column
--   * target_table_name table name of the table that provides the values
--   * target_column column name of the column that provides the values
--   * schema_name optional parameter to defina the schema the target table
--     belongs to, which is necessary if its not in the search_path.
--     Note that target_table_name should never include the schema in it.
-- Return value:
--   Aereal-weighted interpolation of the column values over the geometry
CREATE OR REPLACE
FUNCTION cdb_overlap_sum(geom geometry, target_table_name text, target_column text, schema_name text DEFAULT NULL)
  RETURNS numeric AS
$$
DECLARE
	result numeric;
  qualified_name text;
BEGIN
  IF schema_name IS NULL THEN
    qualified_name := Format('%I', target_table_name);
  ELSE
    qualified_name := Format('%I.%s', schema_name, target_table_name);
  END IF;
  EXECUTE Format('
    SELECT sum(%I*ST_Area(St_Intersection($1, a.the_geom))/ST_Area(a.the_geom))
    FROM %s AS a
    WHERE $1 && a.the_geom
  ', target_column, qualified_name)
  USING geom
  INTO result;
  RETURN result;
END;
$$ LANGUAGE plpgsql;