presto-main/src/main/java/io/prestosql/operator/TransformWork.java/TransformWork/process
class TransformWork:    @Override
    public boolean process()
    {
        checkState(!finished);
        finished = work.process();
        if (!finished) {
            return false;
        }
        result = transformation.apply(work.getResult());
        return true;
    }