How can I find a point inside a polygon in PostGIS?

How can I find a point that is guaranteed to be within a given polygon in PostGIS?

I am aware of the ST_Centroid function. However, the centroid is not always within a polygon, see below:

Furthermore, I would like to avoid using a point that is on the polygon boundary, but rather want one that is inside the boundary (and not inside a hole in donut shaped polygons).

Answer

If you're looking for a PostGIS function that will tell you a point that's inside your polygon then the ST_PointOnSurface function may give you what you need.

SELECT 
   ST_AsText(ST_PointOnSurface('POLYGON((0 0, 0 5, 5 5, 5 0, 0 0))'::geometry));

   st_astext
----------------
 POINT(2.5 2.5)
(1 row)