presto-raptor-legacy/src/main/java/io/prestosql/plugin/raptor/legacy/systemtables/PageListBuilder.java/PageListBuilder/beginRow
class PageListBuilder:    public void beginRow()
    {
        if (pageBuilder.isFull()) {
            pages.add(pageBuilder.build());
            pageBuilder.reset();
        }
        pageBuilder.declarePosition();
        channel = 0;
    }