presto-main/src/main/java/io/prestosql/operator/project/MergePages.java/MergePagesTransformation/isLoaded
class MergePagesTransformation:        private static boolean isLoaded(Page page)
        {
            // TODO: provide better heuristics there, e.g check if last produced page was materialized
            for (int channel = 0; channel < page.getChannelCount(); ++channel) {
                Block block = page.getBlock(channel);
                if (!block.isLoaded()) {
                    return false;
                }
            }

            return true;
        }