presto-main/src/main/java/io/prestosql/split/MappedPageSource.java/MappedPageSource/getNextPage
class MappedPageSource:    @Override
    public Page getNextPage()
    {
        Page nextPage = delegate.getNextPage();
        if (nextPage == null) {
            return null;
        }
        Block[] blocks = Arrays.stream(delegateFieldIndex)
                .mapToObj(nextPage::getBlock)
                .toArray(Block[]::new);
        return new Page(nextPage.getPositionCount(), blocks);
    }