presto-main/src/main/java/io/prestosql/operator/JoinUtils.java/JoinUtils/channelsToPages
class JoinUtils:    public static List<Page> channelsToPages(List<List<Block>> channels)
    {
        ImmutableList.Builder<Page> pagesBuilder = ImmutableList.builder();
        if (!channels.isEmpty()) {
            int pagesCount = channels.get(0).size();
            for (int pageIndex = 0; pageIndex < pagesCount; ++pageIndex) {
                Block[] blocks = new Block[channels.size()];
                for (int channelIndex = 0; channelIndex < channels.size(); ++channelIndex) {
                    blocks[channelIndex] = channels.get(channelIndex).get(pageIndex);
                }
                pagesBuilder.add(new Page(blocks));
            }
        }
        return pagesBuilder.build();
    }