St_split return the original line

Question

I would like to split a linestring with postgis.

My function :

  @distance_b_to_a = Track.find_by_sql(
  ["SELECT
    ST_Split(line, ST_Closestpoint(pta,line))  AS dst_line
    FROM (
      SELECT
        'SRID=4326;LINESTRING(1.391991 46.441430,1.506078 46.556788)'::geometry line,
        'SRID=4326;POINT(1.396636962890625 46.442352066959174)'::geometry pta,
    ) data"
  ])

and my show :

  <p>Test : <%= @distance_b_to_a.first.dst_line %></p>

return my original linestring :

Test : GEOMETRYCOLLECTION (LINESTRING (1.391991 46.44143, 1.506078 46.556788))

What's wrong ?

Answer 1

ST_Split returns a collection of geometries resulting by splitting a geometry, so I'm afraid it's the expected result, since your LineString has only two points.

Consider the following LineString ...

LINESTRING(18.6435 42.5412,18.8440 42.5453,18.846 42.3994)

.. and run this query, which applies ST_Split using the second point from the above mentioned LineString:

WITH j AS (
SELECT 
    'SRID=4326;POINT(18.8440 42.5453)'::GEOMETRY AS pta,
    'SRID=4326;LINESTRING(18.6435 42.5412,18.8440 42.5453,18.846 42.3994)'::GEOMETRY AS line
)
SELECT 
  ST_AsText(
  ST_Split(line,
    ST_ClosestPoint(pta,line))) FROM j

... returning a GeometryCollection with two LineStrings:

                                                st_astext                                                 
----------------------------------------------------------------------------------------------------------
 GEOMETRYCOLLECTION(LINESTRING(18.6435 42.5412,18.844 42.5453),LINESTRING(18.844 42.5453,18.846 42.3994))
(1 Zeile)