{
  "name": "boundary",
  "type": "function",
  "groups": ["GeometryGroup"],
  "description": "Returns the closure of the combinatorial boundary of the geometry (ie the topological boundary of the geometry). For instance, a polygon geometry will have a boundary consisting of the linestrings for each ring in the polygon. Some geometry types do not have a defined boundary, e.g., points or geometry collections, and will return NULL.",
  "arguments": [{
    "arg": "geometry",
    "description": "a geometry"
  }],
  "examples": [{
    "expression": "geom_to_wkt(boundary(geom_from_wkt('Polygon((1 1, 0 0, -1 1, 1 1))')))",
    "returns": "'LineString(1 1,0 0,-1 1,1 1)'"
  }, {
    "expression": "geom_to_wkt(boundary(geom_from_wkt('LineString(1 1,0 0,-1 1)')))",
    "returns": "'MultiPoint ((1 1),(-1 1))'"
  }],
  "tags": ["polygon", "linestrings", "instance", "defined", "combinatorial", "topological", "points", "consisting", "collections", "types", "return", "ring", "boundary", "closure"]
}