after st_union() PgSQL returns a note:
NOTE: TopologyException: side location conflict at 226.001 39.5158
And in the results there are two rows with empty geometry and other rows are ok.
What causes that problem? How to solve it?
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I've found the problem. It was a self intersecting polygon.
ST_IsValidReason to find it.
The solution arrived in PostGIS 2.0, ST_IsValid() and ST_MakeValid(). You can ask PostGIS to fix broken geometries. You must be careful with it tough it the source geometry's integrity is important for some reason.
Googling found this related post: http://lists.refractions.net/pipermail/jts-devel/2009-March/002939.html
So it seems your features may be suffering from rounding errors, and it may be solvable by cleaning up the geometry a bit.
It might be worth creating a minimum reproducible case, and post a message on the postgis mailing list.