SpatialDB Advisor

Vectorization: Exploding a linestring or polygon into individual vectors in PostGIS

I find having a vectorising function in my database kit-bag almost indispensable in my work. I have written and revised my Oracle PL/SQL functions that do this many times over the years and so, when I first started using PostGIS, I decided to learn how to program in PL/pgSQL by implementing something familiar: a GetVector() function.

Now, my PostgreSQL/PostGIS is not bad, but I am always willing to learn from others who are more expert than me: so don’t take the following function (which , for example, doesn’t handle Curves as they are still in their infancy in PostGIS) as being perfect or correct in every manner.

Firstly, we need to create two data types that we can use.

A Coordinate Type

CREATE TYPE coordtype AS (
    x double precision,
    y double precision,
    z double precision,
    m double precision
);
Query returned successfully with no result in 78 ms.

A Vector Type based on the Coordinate Type

CREATE TYPE vectortype AS (
    startcoord coordtype,
    endcoord coordtype
);
Query returned successfully with no result in 16 ms.

What I tried to do in the following GetVector() function is handle all geometry types in the one function. Initially I used one cursor per geometry type (and a single refCursor for processing) but in this implementation I have chosen to use a single cursor with UNION ALLs between the SQL that processes the vertices of each geometry type. I am hoping that the PostgreSQL query optimiser does its equivalent of Oracle’s partition pruning. Perhaps someone may test this aspect and report if it is not performing as fast as one would hope.

CREATE OR REPLACE FUNCTION ST_GetVector(p_geometry geometry)
  RETURNS SETOF VectorType IMMUTABLE  
AS $$
DECLARE
    v_GeometryType   varchar(1000);
    v_rec            RECORD;
    v_vector         VectorType;
    v_start          CoordType;
    v_end            CoordType;
    c_points CURSOR ( p_geom         geometry,
                      p_Geometrytype text ) 
    IS
      SELECT ST_X(sp) as sx,ST_Y(sp) as sy,ST_Z(sp) as sz,ST_M(sp) as sm,
             ST_X(ep) as ex,ST_Y(ep) as ey,ST_Z(ep) as ez,ST_M(ep) as em
       FROM ( SELECT pointn(p_geom, generate_series(1, npoints(p_geom)-1)) as sp,
                     pointn(p_geom, generate_series(2, npoints(p_geom)  )) as ep
               WHERE ( p_GeometryType = 'ST_LineString' ) 
            ) AS linestring
      UNION ALL
      SELECT ST_X(sp),ST_Y(sp),ST_Z(sp),ST_M(sp),ST_X(ep),ST_Y(ep),ST_Z(ep),ST_M(ep)
        FROM ( SELECT ST_PointN(b.geom, generate_series(1, ST_NPoints(b.geom)-1)) as sp,
                      ST_PointN(b.geom, generate_series(2, ST_NPoints(b.geom)  )) as ep
                 FROM (SELECT ST_GeometryN(p_geom,generate_series(1,ST_NumGeometries(p_geom))) as geom) as b
                        WHERE ( p_GeometryType = 'ST_MultiLineString' ) 
             ) AS multiline
      UNION ALL
      SELECT ST_X(sp),ST_Y(sp),ST_Z(sp),ST_M(sp),ST_X(ep),ST_Y(ep),ST_Z(ep),ST_M(ep)
        FROM ( SELECT ST_PointN(a.geom, generate_series(1, ST_NPoints(a.geom)-1)) as sp,
                      ST_PointN(a.geom, generate_series(2, ST_NPoints(a.geom)  )) as ep
                 FROM ( SELECT ST_ExteriorRing(p_geom) as geom
                         WHERE ( p_GeometryType = 'ST_Polygon' )
                        UNION ALL
                        SELECT ST_InteriorRingN(p_geom,generate_series(1,ST_NumInteriorRings(p_geom))) as geom
                         WHERE ( p_GeometryType = 'ST_Polygon' )
                      ) a
             ) as polygon
      UNION ALL
      SELECT ST_X(sp),ST_Y(sp),ST_Z(sp),ST_M(sp),ST_X(ep),ST_Y(ep),ST_Z(ep),ST_M(ep)
        FROM ( SELECT ST_pointn(a.geom, generate_series(1, ST_npoints(a.geom)-1)) as sp,
                      ST_pointn(a.geom, generate_series(2, ST_npoints(a.geom)  )) as ep
                 FROM ( SELECT ST_exteriorring(b.geom) as geom
                          FROM (select ST_geometryn(p_geom,generate_series(1,ST_numgeometries(p_geom))) as geom ) b    
                                 WHERE ( p_GeometryType = 'ST_MultiPolygon' )
                        UNION ALL
                        SELECT ST_InteriorRingn(c.geom,generate_series(1,ST_NumInteriorRings(c.geom))) as geom
                          FROM (select ST_GeometryN(p_geom,generate_series(1,ST_NumGeometries(p_geom))) as geom ) c   
                                 WHERE ( p_GeometryType = 'ST_MultiPolygon' )
                      ) a
                    ) as multipoly;
Begin
    If ( p_geometry is NULL ) Then
      return;
    End If;
    v_GeometryType := ST_GeometryType(p_geometry);
    If    ( v_GeometryType in ('ST_Point','ST_MultiPoint') ) Then
      return;
    End If;
    IF ( v_GeometryType = 'ST_Geometry' ) THEN
      -- Could be anything... use native PostGIS function
      v_GeometryType = GeometryType(p_geometry);
      IF ( v_geometryType = 'GEOMETRYCOLLECTION' ) THEN
         FOR v_geom IN 1..ST_NumGeometries(p_geometry) LOOP
            FOR v_rec IN SELECT * FROM ST_GetVector(ST_GeometryN(p_geometry,v_geom)) LOOP
   	        RETURN NEXT v_rec;
   	    END LOOP;
         END LOOP;
      ELSE
         -- Probably CURVED something...
         RETURN;
      END IF;
    END IF;
    IF ( v_geometryType NOT IN ('ST_Geometry','GEOMETRYCOLLECTION') ) THEN
      OPEN c_points(p_geometry,v_Geometrytype);
      LOOP
        FETCH c_points INTO 
              v_start.x, v_start.y, v_start.z, v_start.m,
              v_end.x,   v_end.y,   v_end.z,   v_end.m;
        v_vector.startcoord := v_start;
        v_vector.endcoord   := v_end;
        EXIT WHEN NOT FOUND;
        RETURN NEXT v_vector;
      END LOOP;
      CLOSE c_points;
    END IF;
end;
$$ LANGUAGE 'plpgsql';
Query returned successfully with no result in 156 ms.

Now, so that we can use this function in nested table Select statements we construct a “wrapper”.

CREATE OR REPLACE FUNCTION ST_GetVectorSQL(p_geometry geometry)
  RETURNS SETOF VectorType IMMUTABLE 
AS
$$
	SELECT * FROM ST_GetVector($1);
$$
LANGUAGE 'sql';
Query returned successfully with no result in 31 ms.

Now let’s conduct some simple tests.

select * from ST_GetVector('GEOMETRYCOLLECTION(POINT(2 3 4),LINESTRING(2 3 4,3 4 5))'::geometry);

select * from ST_GetVector('LINESTRING(0 0, 1 1, 2 2, 3 3)'::geometry) as geom;

select * from ST_GetVector('MULTILINESTRING((0 0,1 1,1 2),(2 3,3 2,5 4))'::geometry) As GV;

select * from ST_GetVector('POLYGON((326454.7 5455793.7,326621.3 5455813.7,326455.4 5455796.6,326454.7 5455793.7))'::geometry);

select * from ST_GetVector('MULTIPOLYGON(((326454.7 5455793.7,326621.3 5455813.7,326455.4 5455796.6,326454.7 5455793.7)),((326771.6 5455831.6,326924.1 5455849.9,326901.9 5455874.2,326900.7 5455875.8,326888.9 5455867.3,326866 5455853.1,326862 5455851.2,326847.4 5455845.8,326827.7 5455841.2,326771.6 5455831.6)))'::geometry);

Table Tests
Now we can test our function against some real tables and SQL.

DROP  TABLE v_polygons;

Query returned successfully with no result in 16 ms.

CREATE TABLE v_polygons
(
  gid serial NOT NULL primary key
)
WITH (
  OIDS=FALSE
);

NOTICE:  CREATE TABLE will create implicit sequence "v_polygons_gid_seq" for serial column "v_polygons.gid"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "v_polygons_pkey" for table "v_polygons"
Query returned successfully with no result in 125 ms.

ALTER TABLE v_polygons OWNER TO postgres;

Query returned successfully with no result in 16 ms.

SELECT addGeometryColumn('public','v_polygons','geom','28355','MULTIPOLYGON','2'); 
Total query runtime: 78 ms.
1 rows retrieved.

ALTER TABLE v_polygons DROP CONSTRAINT enforce_geotype_geom;

Query returned successfully with no result in 16 ms.

ALTER TABLE v_polygons ADD  CONSTRAINT enforce_geotype_geom CHECK ((geometrytype(geom) = ANY (ARRAY['MULTIPOLYGON'::text, 'POLYGON'::text])) OR geom IS NULL);

INSERT INTO v_polygons (geom) 
VALUES 
(ST_GeomFromText('MULTIPOLYGON(((326454.7 5455793.7, 326621.3 5455813.7, 326455.4 5455796.6, 326454.7 5455793.7)), ((326771.6 5455831.6, 326924.1 5455849.9, 326901.9 5455874.2, 326900.7 5455875.8, 326888.9 5455867.3, 326866 5455853.1, 326862 5455851.2, 326847.4 5455845.8, 326827.7 5455841.2, 326771.6 5455831.6)))',28355)),
(ST_PolygonFromText('POLYGON((326667.8 5455760.3, 326669.7 5455755.6, 326688.1 5455766.0, 326685.3 5455770.5, 326667.8 5455760.3))',28355));

SELECT v_polygons.gid, 
       (ST_GetVectorSQL(v_polygons.geom)).*
  FROM v_polygons;

Let’s visualise this by moving the resultant vectors parallel 10 meters from the original polygon boundaries.

We will use Regina Obe’s upgis_lineshift function to move individual polygon vectors to the right by 10m.

CREATE OR REPLACE FUNCTION upgis_lineshift(centerline geometry, dist double precision)
  RETURNS geometry AS
  $$
  DECLARE
        delx float;
        dely float;
        x0 float;
        y0 float;
        x1 float;
        y1 float;
        az float;
        dir integer;
        line geometry;
  BEGIN
        az := ST_Azimuth (ST_StartPoint(centerline), ST_EndPoint(centerline));
        dir := CASE WHEN az < pi() THEN -1 ELSE 1 END;
        delx := ABS(COS(az)) * dist * dir;
        dely := ABS(SIN(az)) * dist * dir;

        IF az > pi()/2 AND az < pi() OR az > 3 * pi()/2 THEN
                line := ST_Translate(centerline, delx, dely) ;
        ELSE
                line := ST_Translate(centerline, -delx, dely);
        END IF;

        RETURN line;
  END;
  $$
  LANGUAGE 'plpgsql' IMMUTABLE;
  COMMENT ON FUNCTION upgis_lineshift(geometry, double precision) IS 'Takes a 2D line string and shifts it dist units along  the perpendicular defined by the straight line between the start and end point.
Convention: (right is positive and left is negative. Right being defined as to right of observer standing at start point and looking down the end point)';

Query returned successfully with no result in 78 ms.

Then we need a table to hold the vectors we create and shift.

DROP   TABLE polygon_vectors;

Query returned successfully with no result in 31 ms.

CREATE TABLE polygon_vectors
(
  gid serial NOT NULL primary key
)
WITH (
  OIDS=FALSE
);

NOTICE:  CREATE TABLE will create implicit sequence "polygon_vectors_gid_seq" for serial column "polygon_vectors.gid"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "polygon_vectors_pkey" for table "polygon_vectors"
Query returned successfully with no result in 125 ms.

ALTER TABLE v_polygons OWNER TO postgres;

Query returned successfully with no result in 16 ms.

SELECT addGeometryColumn('public','polygon_vectors','geom','28355','LINESTRING','2'); 

Finally, let’s create, shift and write the vectors.

INSERT INTO polygon_vectors (geom)
SELECT upgis_lineshift(ST_SetSRID(ST_MakeLine(ST_MakePoint((startcoord).x,(startcoord).y)::geometry, 
                                              ST_MakePoint((endcoord).x,  (endcoord).y)::geometry),
                                  28355),
                       10)
  FROM (SELECT (ST_GetVector(v_polygons.geom)).*
          FROM v_polygons ) as v;

Query returned successfully: 16 rows affected, 15 ms execution time.

SELECT COUNT(*)
  FROM polygon_vectors;

Total query runtime: 16 ms.
1 rows retrieved.

Finally, let’s visualise the polygons and the shifted vectors.

I hope this function, and article, is of use to someone.