presto-main/src/main/java/io/prestosql/operator/project/MergePages.java/MergePagesTransformation/appendPage
class MergePagesTransformation:        private void appendPage(Page page)
        {
            pageBuilder.declarePositions(page.getPositionCount());
            for (int channel = 0; channel < types.size(); channel++) {
                Type type = types.get(channel);
                for (int position = 0; position < page.getPositionCount(); position++) {
                    type.appendTo(page.getBlock(channel), position, pageBuilder.getBlockBuilder(channel));
                }
            }
        }