t some <span class="productname">PostgreSQL</span>
    extensions.
   </p><p>
    This chapter will on occasion refer to examples found in <a class="xref" href="tutorial-sql.html" title="Chapter 2. The SQL Language">Chapter 2</a> to change or improve them, so it will be
    useful to have read that chapter.  Some examples from
    this chapter can also be found in
    <code class="filename">advanced.sql</code> in the tutorial directory.  This
    file also contains some sample data to load, which is not
