[postgis-devel] dump(geometry)

[strk at refractions.net]

I've been researching and implementing a geometry dumper
capable of removing the dead-end constraint on GEOMETRYCOLLECTION.

It is a set-returning function (SRF) returning each of the
input geometry components togheter with it's internal "path".

The returned row is of the custom geometry_dump type:

CREATE TYPE geometry_dump AS (path integer[], geom geometry);

I've made it work on 'single' geometry types as well, using
an empty 'path' for them.

The path expresses the component position inside it's original
structure, to allow further analisys using SQL.

Single geometries are returned with an empty array as path, so
you can use a single select to completely explode a relation
containing mixed geometry types.

I'm not sure about the use of arrays, but I think returning
a SETOF 'geometry_dump' instead of a SETOF 'geometry' gives
more power to the users.

Also, this is only tested against postgresql 8.0.0.
Tests on previous postgresql versions are *very* welcome.

The (alpha) code is in lwgeom_dump.c.
The file also contains SQL enabler code to be manually added
to lwpostgis.sql.in.

Carl: happy hacking ;)


Example run:

strk=# select id,astext(geom) from test1 where id = 277;
 id  |                                                                                                                                                               astext                                                                                                                                                     
-----+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 277 | GEOMETRYCOLLECTION(LINESTRING(0 0,1 1),LINESTRING(0 -3,1 10),POINT(2 2),GEOMETRYCOLLECTION(LINESTRING(0 0,1 1),LINESTRING(0 -3,1 10),POINT(2 2)),GEOMETRYCOLLECTION(LINESTRING(0 0,1 1),LINESTRING(0 -3,1 10),POINT(2 2),GEOMETRYCOLLECTION(LINESTRING(0 0,1 1),LINESTRING(0 -3,1 10),POINT(2 2))),LINESTRING(0 0,2 2,10 10,10 50))
(1 row)


strk=# select id,astext(geom(dump(geom))),path(dump(geom)) from test1
strk-# where id = 277;
 id  |             astext              |  path
-----+---------------------------------+---------
 277 | LINESTRING(0 0,1 1)             | {1}
 277 | LINESTRING(0 -3,1 10)           | {2}
 277 | POINT(2 2)                      | {3}
 277 | LINESTRING(0 0,1 1)             | {4,1}
 277 | LINESTRING(0 -3,1 10)           | {4,2}
 277 | POINT(2 2)                      | {4,3}
 277 | LINESTRING(0 0,1 1)             | {5,1}
 277 | LINESTRING(0 -3,1 10)           | {5,2}
 277 | POINT(2 2)                      | {5,3}
 277 | LINESTRING(0 0,1 1)             | {5,4,1}
 277 | LINESTRING(0 -3,1 10)           | {5,4,2}
 277 | POINT(2 2)                      | {5,4,3}
 277 | LINESTRING(0 0,2 2,10 10,10 50) | {6}
(13 rows)

Comment welcome.

--strk;

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