presto-main/src/main/java/io/prestosql/operator/PageBuffer.java/PageBuffer/add
class PageBuffer:    public void add(Page page)
    {
        checkState(isEmpty(), "page buffer is not empty");
        checkState(!isFinished(), "page buffer is finished");
        requireNonNull(page, "page is null");

        if (page.getPositionCount() == 0) {
            return;
        }

        this.page = page;
    }