PlaidCloud expressions and filters provide use of most non-administrative PostGIS methods. PostGIS methods are accessed by prefixing the standard method name with
ST_UnaryUnion(geometry geom, float8 gridSize = -1);
func.ST_UnaryUnion(geometry geom, float8 gridSize = -1);
PostGIS Official Documentation for this method can be found here.
Additional capabilities and usage examples can be found in the PostGIS documentation