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 ST_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_Vectorize(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 )
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 ST_PointN(p_geom, generate_series(1, ST_NPoints(p_geom)-1)) as sp,
ST_PointN(p_geom, generate_series(2, ST_NPoints(p_geom) )) as ep
WHERE ST_GeometryType(p_geom) = ‘ST_LineString’
UNION ALL
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
WHERE ST_GeometryType(p_geom) = ‘ST_MultiLineString’
) as b
UNION ALL
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
UNION ALL
SELECT ST_InteriorRingN(p_geom,generate_series(1,ST_NumInteriorRings(p_geom))) as geom
) a
WHERE ST_GeometryType(p_geom) = ‘ST_Polygon’
UNION ALL
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) as b
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) as c
) a
WHERE ST_GeometryType(p_geom) = ‘ST_MultiPolygon’
) as f;

Begin
If ( p_geometry is NULL ) Then
return;
End If;
v_GeometryType := ST_GeometryType(p_geometry);
— RAISE NOTICE ‘ST_GeometryType %’, v_GeometryType;
IF ( v_GeometryType in (‘ST_Point’,‘ST_MultiPoint’,‘ST_Geometry’) ) Then
return;
END IF;
IF ( v_GeometryType IN (‘ST_GeometryCollection’) ) THEN
FOR v_geom IN 1..ST_NumGeometries(p_geometry) LOOP
FOR v_rec IN SELECT * FROM ST_Vectorize(ST_GeometryN(p_geometry,v_geom)) LOOP
RETURN NEXT v_rec;
END LOOP;
END LOOP;
ELSE
OPEN c_points(p_geometry);
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’;
CREATE OR REPLACE FUNCTION ST_VectorizeSQL(p_geometry geometry)
RETURNS SETOF VectorType IMMUTABLE

AS

$$
SELECT * FROM ST_Vectorize($1);

$$

LANGUAGE ‘sql’;

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);
```

startcoord coordtype |
endcoord coordtype |

(2,3,4,) |
(3,4,5,) |

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

startcoord coordtype |
endcoord coordtype |

(0,0,,) |
(1,1,,) |

(1,1,,) |
(2,2,,) |

(2,2,,) |
(3,3,,) |

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

startcoord coordtype |
endcoord coordtype |

(0,0,,) |
(1,1,,) |

(1,1,,) |
(1,2,,) |

(2,3,,) |
(3,2,,) |

(3,2,,) |
(5,4,,) |

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

startcoord coordtype |
endcoord coordtype |

(326454.7,5455793.7,,) |
(326621.3,5455813.7,,) |

(326621.3,5455813.7,,) |
(326455.4,5455796.6,,) |

(326455.4,5455796.6,,) |
(326454.7,5455793.7,,) |

```
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);
```

startcoord coordtype |
endcoord coordtype |

(326454.7,5455793.7,,) |
(326621.3,5455813.7,,) |

(326621.3,5455813.7,,) |
(326455.4,5455796.6,,) |

(326455.4,5455796.6,,) |
(326454.7,5455793.7,,) |

(326771.6,5455831.6,,) |
(326924.1,5455849.9,,) |

(326924.1,5455849.9,,) |
(326901.9,5455874.2,,) |

(326901.9,5455874.2,,) |
(326900.7,5455875.8,,) |

(326900.7,5455875.8,,) |
(326888.9,5455867.3,,) |

(326888.9,5455867.3,,) |
(326866,5455853.1,,) |

(326866,5455853.1,,) |
(326862,5455851.2,,) |

(326862,5455851.2,,) |
(326847.4,5455845.8,,) |

(326847.4,5455845.8,,) |
(326827.7,5455841.2,,) |

(326827.7,5455841.2,,) |
(326771.6,5455831.6,,) |

**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.
```

addgeometrycolumn text |

public.v_polygons.geom SRID:28355 TYPE:MULTIPOLYGON DIMS:2 |

```
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;
```

oid integer |
startcoord coordtype |
endcoord coordtype |

1 |
(326454.7,5455793.7,,) |
(326621.3,5455813.7,,) |

1 |
(326621.3,5455813.7,,) |
(326455.4,5455796.6,,) |

1 |
(326455.4,5455796.6,,) |
(326454.7,5455793.7,,) |

1 |
(326771.6,5455831.6,,) |
(326924.1,5455849.9,,) |

1 |
(326924.1,5455849.9,,) |
(326901.9,5455874.2,,) |

1 |
(326901.9,5455874.2,,) |
(326900.7,5455875.8,,) |

1 |
(326900.7,5455875.8,,) |
(326888.9,5455867.3,,) |

1 |
(326888.9,5455867.3,,) |
(326866,5455853.1,,) |

1 |
(326866,5455853.1,,) |
(326862,5455851.2,,) |

1 |
(326862,5455851.2,,) |
(326847.4,5455845.8,,) |

1 |
(326847.4,5455845.8,,) |
(326827.7,5455841.2,,) |

1 |
(326827.7,5455841.2,,) |
(326771.6,5455831.6,,) |

2 |
(326667.8,5455760.3,,) |
(326669.7,5455755.6,,) |

2 |
(326669.7,5455755.6,,) |
(326688.1,5455766,,) |

2 |
(326688.1,5455766,,) |
(326685.3,5455770.5,,) |

2 |
(326685.3,5455770.5,,) |
(326667.8,5455760.3,,) |

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');
```

addgeometrycolumn text |

public.polygon_vectors.geom SRID:28355 TYPE:LINESTRING DIMS: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.