Thread: Breaking Path/Polygon Data into Pieces

Hi,

2 weeks ago, a user in -tr-genel asked for a function to break
path/polygon type data into pieces. He also told that, it creates a
bottleneck in the network traffic when they try to receive rows with
polygon data of thousands of nodes, while it's enough for them to
have polygons partially.

AFAIK, there doesn't exist such functions in PostgreSQL. (Please
correct me if I'm wrong.) For this purpose, I've coded two simple
C procedures:

  polygon part(polygon, offset, limit)
  path    part(path, offset, limit)

I thought it would be nice to see these functions in PostgreSQL and
wanted to ask for other users (and developers) ideas.


Regards.

On Mon, Feb 27, 2006 at 08:41:52PM +0200, Volkan YAZICI wrote:
> 2 weeks ago, a user in -tr-genel asked for a function to break
> path/polygon type data into pieces. He also told that, it creates a
> bottleneck in the network traffic when they try to receive rows with
> polygon data of thousands of nodes, while it's enough for them to
> have polygons partially.
>
> AFAIK, there doesn't exist such functions in PostgreSQL. (Please
> correct me if I'm wrong.) For this purpose, I've coded two simple
> C procedures:
>
>   polygon part(polygon, offset, limit)
>   path    part(path, offset, limit)

PostGIS has geometry accessors that might work.  You'd need to be
using PostGIS geometry types instead of the PostgreSQL types.

http://postgis.refractions.net/docs/ch06.html

Are the following examples anything like what the user in tr-general
was looking for?

postgis=> SELECT AsText(geom) FROM foo;
                          astext
----------------------------------------------------------
 POLYGON((0 0,10 0,10 10,0 10,0 0),(1 1,9 1,9 9,1 9,1 1))
(1 row)

postgis=> SELECT AsText(ExteriorRing(geom)) FROM foo;
               astext
-------------------------------------
 LINESTRING(0 0,10 0,10 10,0 10,0 0)
(1 row)

postgis=> SELECT AsText(InteriorRingN(geom, 1)) FROM foo;
             astext
---------------------------------
 LINESTRING(1 1,9 1,9 9,1 9,1 1)
(1 row)

postgis=> SELECT n, AsText(PointN(ring, n))
postgis-> FROM (SELECT ExteriorRing(geom) AS ring FROM foo) AS s,
postgis->      generate_series(1, 2) AS g(n);
 n |   astext
---+-------------
 1 | POINT(0 0)
 2 | POINT(10 0)
(2 rows)

postgis=> SELECT AsText(MakeLine(PointN(ring, n)))
postgis-> FROM (SELECT ExteriorRing(geom) AS ring FROM foo) AS s,
postgis->      generate_series(1, 2) AS g(n);
        astext
----------------------
 LINESTRING(0 0,10 0)
(1 row)

postgis=> SELECT AsText(line_substring(ExteriorRing(geom), 0, 0.25))
postgis-> FROM foo;
        astext
----------------------
 LINESTRING(0 0,10 0)
(1 row)

--
Michael Fuhr

On Feb 27 03:10, Michael Fuhr wrote:
> PostGIS has geometry accessors that might work.  You'd need to be
> using PostGIS geometry types instead of the PostgreSQL types.
>
> http://postgis.refractions.net/docs/ch06.html
>
> Are the following examples anything like what the user in tr-general
> was looking for?

Yes. As I understand from your examples, they're quite efficient for
related purposes I mentioned. But wouldn't it worth adding a simple
part() function for this? Because, with this feature, people won't need
to use another project for such a basic operation on path and polygon
types which are supported by PostgreSQL by default.


Regards.