[postgis-users] st_union returning null on various (valid) sets of polygons/multipolygons

[G. Allegri]

I'm trying to apply ST_Union on a set of geometries (multypolygons,
most of which containing a single geometry), to test the CascadedUnion
performance, but the most of times it returns a null result. I haven't
been able to find a rule: sometimes it seems to be the number of
geometries, sometimes the order, etc.
Could someone replicate it? Here is a small selection of my layer (it
contains already validated geometries):
http://www.giovanniallegri.it/share/postgis_selection.zip

thanks a lot,
Giovanni