presto-main/src/main/java/io/prestosql/operator/ValuesOperator.java/ValuesOperator/getOutput
class ValuesOperator:    @Override
    public Page getOutput()
    {
        if (!pages.hasNext()) {
            return null;
        }
        Page page = pages.next();
        if (page != null) {
            operatorContext.recordProcessedInput(page.getSizeInBytes(), page.getPositionCount());
        }
        return page;
    }