<entry>Comment</entry>
       </row>
      </thead>

      &features-supported;

     </tgroup>
    </informaltable>
   </para>
  </sect1>

  <sect1 id="unsupported-features-sql-standard">
   <title>Unsupported Features</title>

   <para>
    The following features defined in <acronym>SQL:2023</acronym> are not
    implemented in this release of
    <productname>PostgreSQL</productname>. In a few cases, equivalent
    functionality is available.

    <informaltable>
     <tgroup cols="4">
      <colspec colname="col1" colwidth="1.5*"/>
      <colspec colname="col2" colwidth="1*"/>
      <colspec colname="col3" colwidth="7*"/>
      <colspec colname="col4" colwidth="3*"/>
      <thead>
       <row>
        <entry>Identifier</entry>
        <entry>Core?</entry>
        <entry>Description</entry>
        <entry>Comment</entry>
       </row>
      </thead>

      &features-unsupported;

     </tgroup>
    </informaltable>
   </para>
  </sect1>

  <sect1 id="xml-limits-conformance">
   <title>XML Limits and Conformance to SQL/XML</title>

   <indexterm>
    <primary>SQL/XML</primary>
    <secondary>limits and conformance</secondary>
   </indexterm>

   <para>
    Significant revisions to the XML-related specifications in ISO/IEC 9075-14
    (SQL/XML) were introduced with SQL:2006.
    <productname>PostgreSQL</productname>'s implementation of the XML data
    type and related functions largely follows the earlier 2003 edition,
    with some borrowing from later editions.  In particular:
    <itemizedlist>
     <listitem>
      <para>
       Where the current standard provides a family of XML data types
       to hold <quote>document</quote> or <quote>content</quote> in
       untyped or XML Schema-typed variants, and a type
       <type>XML(SEQUENCE)</type> to hold arbitrary pieces of XML content,
       <productname>PostgreSQL</productname> provides the single
       <type>xml</type> type, which can hold <quote>document</quote> or
       <quote>content</quote>.  There is no equivalent of the
       standard's <quote>sequence</quote> type.
      </para>
     </listitem>

     <listitem>
      <para>
       <productname>PostgreSQL</productname> provides two functions
       introduced in SQL:2006, but in variants that use the XPath 1.0
       language, rather than XML Query as specified for them in the
       standard.
      </para>
     </listitem>

     <listitem>
      <para>
       <productname>PostgreSQL</productname> does not support the
       <literal>RETURNING CONTENT</literal> or <literal>RETURNING SEQUENCE</literal>
       clauses, functions which are defined to have these in the specification
       are implicitly returning content.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    This section presents some of the resulting differences you may encounter.
   </para>

   <sect2 id="functions-xml-limits-xpath1">
    <title>Queries Are Restricted to XPath 1.0</title>

    <para>
     The <productname>PostgreSQL</productname>-specific functions
     <function>xpath()</function> and <function>xpath_exists()</function>
     query XML documents using the XPath language.
     <productname>PostgreSQL</productname> also provides XPath-only variants
     of the standard functions <function>XMLEXISTS</function> and
     <function>XMLTABLE</function>, which officially use
     the XQuery language. For all of these functions,
     <productname>PostgreSQL</productname> relies on the
     <application>libxml2</application> library, which provides only XPath 1.0.
    </para>

    <para>
     There is a strong connection between the XQuery language and XPath
     versions 2.0 and later: any expression that is syntactically valid and
     executes successfully in both produces the same result (with a minor
     exception for expressions containing numeric character references or
     predefined entity references, which XQuery replaces with the
     corresponding character while XPath leaves them alone).  But there