ALTER TABLE fiumiADD COLUMN bufferGEOMETRY(MultiPolygon,3003,2);UPDATE fiumiSET buffer=ST_Multi(ST_Buffer(geom,100));CREATE TABLE buffer_union AS SELECTST_Union(buffer)::GEOMETRY(MultiPolygon,3003),ROUND((ST_Area(ST_Union(buffer))/1000)::numeric,2)FROM fiumi;ALTER TABLE fiumiDROP COLUMN buffer;