presto-main/src/main/java/io/prestosql/operator/project/MergingPageOutput.java/MergingPageOutput/buffer
class MergingPageOutput:    private void buffer(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));
            }
        }
        if (pageBuilder.isFull()) {
            flush();
        }
    }