PostGIS function to get the side of a point or geometry from a line

postgis_side.sql

-- PostGIS function to get the side of a point or geometry from a line 
-- ***** side function
-- ***** does not support m values. Only the digitizing "orientaiton" of the line.
-- -- inspired by: https://gis.stackexchange.com/a/156585
CREATE OR REPLACE FUNCTION st2_side (
    geom_line geometry,
    geom_other geometry
  )
  RETURNS text AS
$BODY$
with shortestline as(
  select ST_ShortestLine(geom_line, geom_other) as geom
), vec_start as (
  select ST_StartPoint((select shortestline.geom from shortestline)) as pt
), dist_along as(
  select ST_LineLocatePoint(geom_line, vec_start.pt) as dist_along
  from vec_start
), new_line_pt1 as(
  SELECT
    case when dist_along > 0.999 then
      ST_LineInterpolatePoint(geom_line, 0.999)
    when dist_along < 0.001 then
      ST_StartPoint(geom_line)
    else
      vec_start.pt
    end as pt
  from dist_along, vec_start
), new_line_pt2 as(
  SELECT
    case when dist_along > 0.999 then
      vec_start.pt
    when dist_along < 0.001 then
      ST_LineInterpolatePoint(geom_line, 0.001)
    else
      ST_LineInterpolatePoint(geom_line, dist_along + 0.001)
    end as pt
  from dist_along, vec_start
), new_line as(
  select ST_MakeLine(new_line_pt1.pt, new_line_pt2.pt) as new_line
  from new_line_pt1, new_line_pt2
), h as(
  select 
    shortestline.geom vec,
    new_line seg
  from shortestline, vec_start, new_line
), az as(
  select (ST_Azimuth(ST_StartPoint(h.vec), ST_EndPoint(h.vec)) - ST_Azimuth(ST_StartPoint(h.seg), ST_EndPoint(h.seg))) as az
  from h
)
select --az::text
  case when az = 0 or az is null THEN
    'middle'
  when az > PI() or (az<0 and az > (-1*PI())) THEN
    'left'
  ELSE
    'right'
  end
from az
$BODY$
LANGUAGE sql;


-- *** Testing:
SELECT st2_side( ST_GeomFromEWKT('LINESTRING(0 0, 2 2)'), ST_GeomFromEWKT('POINT(1 1)')); -- middle
SELECT st2_side( ST_GeomFromEWKT('LINESTRING(0 0, 2 2)'), ST_GeomFromEWKT('POINT(1 0)')); -- right
SELECT st2_side( ST_GeomFromEWKT('LINESTRING(0 0, 2 2)'), ST_GeomFromEWKT('POINT(0 1)')); -- left
SELECT st2_side( ST_GeomFromEWKT('LINESTRING(0 0, 2 2)'), ST_GeomFromEWKT('POINT(-1 0)')); -- left
SELECT st2_side( ST_GeomFromEWKT('LINESTRING(0 0, 2 2)'), ST_GeomFromEWKT('POINT(0 -1)')); -- right
SELECT st2_side( ST_GeomFromEWKT('LINESTRING(0 0, 2 2)'), ST_GeomFromEWKT('POINT(4 0)')); -- right
SELECT st2_side( ST_GeomFromEWKT('LINESTRING(0 0, 1 0)'), ST_GeomFromEWKT('POINT(0.5 1)')); -- left
SELECT st2_side( ST_GeomFromEWKT('LINESTRING(0 0, 1 0)'), ST_GeomFromEWKT('POINT(0.5 -1)')); -- right