CG_Union — Computes the union of two geometries
geometry CG_Union(
                            geometry
                            geomA
                        , 
                            geometry
                            geomB
                        );
Computes the union of two geometries.
Performed by the SFCGAL module
                 
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                 NOTE: this function returns a geometry representing the union.  | 
            
Availability: 3.5.0
            
            This method needs SFCGAL backend.
        
SELECT CG_Union('POINT(.5 0)', 'LINESTRING(-1 0,1 0)');
                cg_union
                -----------
                LINESTRING(-1 0,0.5 0,1 0)
                (1 row)