public static Expression generateFilterExpression(SearchArgument sarg) {
    return translate(sarg.getExpression(), sarg.getLeaves());
  }
@Test
  public void testIsNullOperand() {
    SearchArgument.Builder builder = SearchArgumentFactory.newBuilder();
    SearchArgument arg = builder.startAnd().isNull("salary", PredicateLeaf.Type.LONG).end().build();

    UnboundPredicate expected = Expressions.isNull("salary");
    UnboundPredicate actual = (UnboundPredicate) HiveIcebergFilterFactory.generateFilterExpression(arg);

    assertEquals(actual.op(), expected.op());
    assertEquals(actual.ref().name(), expected.ref().name());
  }