>  1. find out intersecting pairs into cache (id1,id2,geom1,geom2)  2. update 
> cache
> set geom1 = st_difference(geom1, geom2)  3. update main set geom = c.geom1
> from cache where id1 = id

Hum... Life is never that easy.

That would work if one polygon would always overlap with only one polygon but 
if one polygon is overlapped by two polygons or more the same polygon as two 
differences associated with it in the second step and only the last one will 
remains in the last update...

Pierre
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