presto-main/src/main/java/io/prestosql/operator/WindowOperator.java/WindowOperator/getOutput
class WindowOperator:    @Override
    public Page getOutput()
    {
        if (!outputPages.process()) {
            return null;
        }

        if (outputPages.isFinished()) {
            return null;
        }

        return outputPages.getResult();
    }