Simple point-in-polygon UDF for Amazon Redshift based on http://geospatialpython.com/2011/08/point-in-polygon-2-on-line.html

point_in_polygon.sql

CREATE FUNCTION point_in_polygon(point_x float, point_y float, polygon_wkt varchar(max))
RETURNS boolean IMMUTABLE AS
$$

### begin section copied from http://geospatialpython.com/2011/08/point-in-polygon-2-on-line.html (I modifed to return boolean)
# Improved point in polygon test which includes edge
# and vertex points

def point_in_poly(x,y,poly):

   # check if point is a vertex
   if (x,y) in poly: return True

   # check if point is on a boundary
   for i in range(len(poly)):
      p1 = None
      p2 = None
      if i==0:
         p1 = poly[0]
         p2 = poly[1]
      else:
         p1 = poly[i-1]
         p2 = poly[i]
      if p1[1] == p2[1] and p1[1] == y and x > min(p1[0], p2[0]) and x < max(p1[0], p2[0]):
         return True
      
   n = len(poly)
   inside = False

   p1x,p1y = poly[0]
   for i in range(n+1):
      p2x,p2y = poly[i % n]
      if y > min(p1y,p2y):
         if y <= max(p1y,p2y):
            if x <= max(p1x,p2x):
               if p1y != p2y:
                  xints = (y-p1y)*(p2x-p1x)/(p2y-p1y)+p1x
               if p1x == p2x or x <= xints:
                  inside = not inside
      p1x,p1y = p2x,p2y

   if inside: return True
   else: return False
### end section copied from http://geospatialpython.com/2011/08/point-in-polygon-2-on-line.html

#extremely simplistic parsing, assumes that the Polygon is in WKT format and is a valid simple polygon

vertices_string = [p.split(' ') for p in polygon_wkt[9:-2].split(',')]
vertices = [(float(v[0]), float(v[1])) for v in vertices_string]

return point_in_poly(point_x, point_y, vertices)

$$
LANGUAGE plpythonu;