presto-benchmark/src/main/java/io/prestosql/benchmark/HandTpchQuery1.java/TpchQuery1Operator/getOutput
class TpchQuery1Operator:        @Override
        public Page getOutput()
        {
            // only return a page if the page buffer isFull or we are finishing and the page buffer has data
            if (pageBuilder.isFull() || (finishing && !pageBuilder.isEmpty())) {
                Page page = pageBuilder.build();
                pageBuilder.reset();
                return page;
            }
            return null;
        }