using the text mode to pass queries to the server and query
    results to the client (which is the normal mode), PostgreSQL
    converts all character data passed between the client and the
    server and vice versa to the character encoding of the respective
    end; see <xref linkend="multibyte"/>.  This includes string
    representations of XML values, such as in the above examples.
    This would ordinarily mean that encoding declarations contained in
    XML data can become invalid as the character data is converted
    to other encodings while traveling between client and server,
    because the embedded encoding declaration is not changed.  To cope
    with this behavior, encoding declarations contained in
    character strings presented for input to the <type>xml</type> type
    are <emphasis>ignored</emphasis>, and content is assumed
    to be in the current server encoding.  Consequently, for correct
    processing, character strings of XML data must be sent
    from the client in the current client encoding.  It is the
    responsibility of the client to either convert documents to the
    current client encoding before sending them to the server, or to
    adjust the client encoding appropriately.  On output, values of
    type <type>xml</type> will not have an encoding declaration, and
    clients should assume all data is in the current client
    encoding.
   </para>

   <para>
    When using binary mode to pass query parameters to the server
    and query results back to the client, no encoding conversion
    is performed, so the situation is different.  In this case, an
    encoding declaration in the XML data will be observed, and if it
    is absent, the data will be assumed to be in UTF-8 (as required by
    the XML standard; note that PostgreSQL does not support UTF-16).
    On output, data will have an encoding declaration
    specifying the client encoding, unless the client encoding is
    UTF-8, in which case it will be omitted.
   </para>

   <para>
    Needless to say, processing XML data with PostgreSQL will be less
    error-prone and more efficient if the XML data encoding, client encoding,
    and server encoding are the same.  Since XML data is internally
    processed in UTF-8, computations will be most efficient if the
    server encoding is also UTF-8.
   </para>

   <caution>
    <para>
     Some XML-related functions may not work at all on non-ASCII data
     when the server encoding is not UTF-8.  This is known to be an
     issue for <function>xmltable()</function> and <function>xpath()</function> in particular.
    </para>
   </caution>
   </sect2>

   <sect2 id="datatype-xml-accessing-xml-values">
   <title>Accessing XML Values</title>

   <para>
    The <type>xml</type> data type is unusual in that it does not
    provide any comparison operators.  This is because there is no
    well-defined and universally useful comparison algorithm for XML
    data.  One consequence of this is that you cannot retrieve rows by
    comparing an <type>xml</type> column against a search value.  XML
    values should therefore typically be accompanied by a separate key
    field such as an ID.  An alternative solution for comparing XML
    values is to convert them to character strings first, but note
    that character string comparison has little to do with a useful
    XML comparison method.
   </para>

   <para>
    Since there are no comparison operators for the <type>xml</type>
    data type, it is not possible to create an index directly on a
    column of this type.  If speedy searches in XML data are desired,
    possible workarounds include casting the expression to a
    character string type and indexing that, or indexing an XPath
    expression.  Of course, the actual query would have to be adjusted
    to search by the indexed expression.
   </para>

   <para>
    The text-search functionality in PostgreSQL can also be used to speed
    up full-document searches of XML data.  The necessary