Amazon Redshift will no longer support the use of Python UDFs after June 30, 2026. We will start enforcing it in phases. For more information on the details of Python end of life and migration options, see the [ blog post ](https://aws.amazon.com/blogs/big-data/amazon-redshift-python-user-defined-functions-will-reach-end-of-support-after-june-30-2026/) that was published on June 30, 2025. 

# ST\_Boundary
<a name="ST_Boundary-function"></a>

ST\_Boundary returns the boundary of an input geometry as follows: 
+ If the input geometry is empty (that is, it contains no points) it is returned as is. 
+ If the input geometry is a point or nonempty multipoint, an empty geometry collection is returned.
+ If the input is a linestring or a multilinestring, then a multipoint containing all the points on the boundary is returned. The multipoint might be empty).
+ If the input is a polygon that does not have any interior rings, then a closed linestring representing its boundary is returned.
+ If the input is a polygon that has interior rings, or is a multipolygon, then a multilinestring is returned. The multilinestring contains all the boundaries of all the rings in the areal geometry as closed linestrings.

To determine point equality, ST\_Boundary operates on the 2D projection of the input geometry. If the input geometry is empty, a copy of it is returned in the same dimension as the input. For nonempty 3DM and 4D geometries, their `m` coordinates are dropped. In the special case of 3DZ and 4D multilinestrings, the `z` coordinates of the multilinestring's boundary points are computed as the averages of the distinct z-values of the linestring boundary points with the same 2D projection.

## Syntax
<a name="ST_Boundary-function-syntax"></a>

```
ST_Boundary(geom)
```

## Arguments
<a name="ST_Boundary-function-arguments"></a>

 *geom*   
A value of data type `GEOMETRY` or an expression that evaluates to a `GEOMETRY` type.

## Return type
<a name="ST_Boundary-function-return"></a>

`GEOMETRY`

If *geom* is null, then null is returned.

If *geom* is a `GEOMETRYCOLLECTION`, then an error is returned.

## Examples
<a name="ST_Boundary-function-examples"></a>

The following SQL returns the boundary of the input polygon as a multilinestring. 

```
SELECT ST_AsEWKT(ST_Boundary(ST_GeomFromText('POLYGON((0 0,10 0,10 10,0 10,0 0),(1 1,1 2,2 1,1 1))')));
```

```
st_asewkt
--------------------
 MULTILINESTRING((0 0,10 0,10 10,0 10,0 0),(1 1,1 2,2 1,1 1))
```