presto-raptor-legacy/src/main/java/io/prestosql/plugin/raptor/legacy/util/ConcatPageSource.java/ConcatPageSource/getNextPage
class ConcatPageSource:    @Override
    public Page getNextPage()
    {
        while (true) {
            setup();

            if (current == null) {
                return null;
            }
            if (!current.isFinished()) {
                return current.getNextPage();
            }

            completedBytes += current.getCompletedBytes();
            readTimeNanos += current.getReadTimeNanos();
            current = null;
        }
    }