presto-main/src/main/java/io/prestosql/operator/exchange/PageChannelSelector.java/PageChannelSelector/apply
class PageChannelSelector:    @Override
    public Page apply(Page page)
    {
        requireNonNull(page, "page is null");

        Block[] blocks = new Block[channels.length];
        for (int i = 0; i < channels.length; i++) {
            int channel = channels[i];
            blocks[i] = page.getBlock(channel);
        }
        return new Page(page.getPositionCount(), blocks);
    }