[postgis-users] ST_Difference not working as aspected

[fLaNsch at gmx.de]

Hi postgis-user,

i like to get the difference between n crossing exterior Linerings and n smaller polygons which are within the rings. In Quantum GIS everything works as aspected.

Scenario:

LineRing A
  ________
 (________)

LineRing B
  ________
 (________)

Situation between A and B
  ______________
 (_____(__)_____)

Two smaller Polygons within the Linerings to erase the shared part of LineRing A and B. If i first use ST_Union to dissolve the Polygon Geometry everything works fine with st_difference, but ST_UNION is no option for a dataset > 4 million.  

With the following SQL-Statement, i can extract the shared parts. 
Select (st_dump(st_intersection(a.geom,b.geom))).geom as geom from line b, poly a where st_intersects(a.geom,b.geom)

But how to keep on? I'm stuck. 


BIG THANKS for any kind of assistance!
Simon