<sect1 id="xml2">
 <title>xml2: XML-handling functions</title>
 
 <indexterm zone="xml2">
  <primary>xml2</primary>
 </indexterm>

 <sect2>
  <title>Deprecation notice</title>
  <para>
    From PostgreSQL 8.3 on, there is XML-related
    functionality based on the SQL/XML standard in the core server.
    That functionality covers XML syntax checking and XPath queries,
    which is what this module does as well, and more, but the API is
    not at all compatible.  It is planned that this module will be
    removed in PostgreSQL 8.4 in favor of the newer standard API, so
    you are encouraged to try converting your applications.  If you
    find that some of the functionality of this module is not
    available in an adequate form with the newer API, please explain
    your issue to pgsql-hackers@postgresql.org so that the deficiency
    can be addressed.
  </para>
 </sect2>
  
 <sect2>
  <title>Description of functions</title>
  <para>
   The first set of functions are straightforward XML parsing and XPath queries:
  </para>

  <table>
   <title>Functions</title>
   <tgroup cols="2">
    <tbody>
     <row>
      <entry>
       <programlisting>
        xml_is_well_formed(document) RETURNS bool
       </programlisting>
      </entry>
      <entry>
       <para>
        This parses the document text in its parameter and returns true if the
        document is well-formed XML.  (Note: before PostgreSQL 8.2, this function
        was called xml_valid().  That is the wrong name since validity and
        well-formedness have different meanings in XML.  The old name is still
        available, but is deprecated and will be removed in 8.3.)
       </para>
      </entry>
     </row>
     <row>
      <entry>
       <programlisting>
        xpath_string(document,query) RETURNS text
        xpath_number(document,query) RETURNS float4
        xpath_bool(document,query) RETURNS bool
       </programlisting>
      </entry>
      <entry>
       <para>
        These functions evaluate the XPath query on the supplied document, and
        cast the result to the specified type.
       </para>
      </entry>
     </row>
     <row>
      <entry>
       <programlisting>
        xpath_nodeset(document,query,toptag,itemtag) RETURNS text
       </programlisting>
      </entry>
      <entry>
       <para>
       This evaluates query on document and wraps the result in XML tags. If
       the result is multivalued, the output will look like:
       </para>
       <literal>
        &lt;toptag>
        &lt;itemtag>Value 1 which could be an XML fragment&lt;/itemtag>
        &lt;itemtag>Value 2....&lt;/itemtag>
        &lt;/toptag>
       </literal>
       <para>
        If either toptag or itemtag is an empty string, the relevant tag is omitted.
       </para>
      </entry>
     </row>
     <row>
      <entry>
       <programlisting>
        xpath_nodeset(document,query) RETURNS
       </programlisting>
      </entry>
      <entry>
       <para>
        Like xpath_nodeset(document,query,toptag,itemtag) but text omits both tags.
       </para>
      </entry>
     </row>
     <row>
      <entry>
       <programlisting>
        xpath_nodeset(document,query,itemtag) RETURNS
       </programlisting>
      </entry>
      <entry>
       <para>
        Like xpath_nodeset(document,query,toptag,itemtag) but text omits toptag.
       </para>
      </entry>
     </row>
     <row>
      <entry>
       <programlisting>
        xpath_list(document,query,seperator) RETURNS text
       </programlisting>
      </entry>
      <entry>
       <para>
        This function returns multiple values seperated by the specified
        seperator, e.g. Value 1,Value 2,Value 3 if seperator=','.
       </para>
      </entry>
     </row>
     <row>
      <entry>
       <programlisting>
        xpath_list(document,query) RETURNS text
       </programlisting>
      </entry>
      <entry>
       This is a wrapper for the above function that uses ',' as the seperator.
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>
 </sect2>


 <sect2>
  <title><literal>xpath_table</literal></title>
  <para>
   This is a table function which evaluates a set of XPath queries on
   each of a set of documents and returns the results as a table. The
   primary key field from the original document table is returned as the
   first column of the result so that the resultset from xpath_table can
   be readily used in joins.
  </para>
  <para>
   The function itself takes 5 arguments, all text.
  </para>
  <programlisting>
   xpath_table(key,document,relation,xpaths,criteria)
  </programlisting>
  <table>
   <title>Parameters</title>
   <tgroup cols="2">
    <tbody>
     <row>
      <entry><literal>key</literal></entry>
      <entry>
       <para>
        the name of the "key" field - this is just a field to be used as
        the first column of the output table i.e. it identifies the record from
        which each output row came (see note below about multiple values).
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>document</literal></entry>
      <entry>
       <para>
        the name of the field containing the XML document
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>relation</literal></entry>
      <entry>
       <para>
        the name of the table or view containing the documents
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>xpaths</literal></entry>
      <entry>
       <para>
        multiple xpath expressions separated by <literal>|</literal>
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>criteria</literal></entry>
      <entry>
       <para>
        The contents of the where clause. This needs to be specified,
        so use "true" or "1=1" here if you want to process all the rows in the
        relation.
       </para>
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   NB These parameters (except the XPath strings) are just substituted
   into a plain SQL SELECT statement, so you have some flexibility - the
   statement is
  </para>

  <para>
   <literal>
   SELECT &lt;key>,&lt;document> FROM &lt;relation> WHERE &lt;criteria>
   </literal>
  </para>

  <para>
   so those parameters can be *anything* valid in those particular
   locations. The result from this SELECT needs to return exactly two
   columns (which it will unless you try to list multiple fields for key
   or document). Beware that this simplistic approach requires that you
   validate any user-supplied values to avoid SQL injection attacks.
  </para>
 
  <para>
   Using the function
  </para>
 
  <para>
   The function has to be used in a FROM expression. This gives the following
   form:
  </para>
 
  <programlisting>
SELECT * FROM
xpath_table('article_id', 
        'article_xml',
        'articles', 
        '/article/author|/article/pages|/article/title',
        'date_entered > ''2003-01-01'' ') 
AS t(article_id integer, author text, page_count integer, title text);
  </programlisting>

  <para>
   The AS clause defines the names and types of the columns in the
   virtual table. If there are more XPath queries than result columns,
   the extra queries will be ignored. If there are more result columns
   than XPath queries, the extra columns will be NULL.
  </para>

  <para>
   Note that I've said in this example that pages is an integer.  The
   function deals internally with string representations, so when you say
   you want an integer in the output, it will take the string
   representation of the XPath result and use PostgreSQL input functions
   to transform it into an integer (or whatever type the AS clause
   requests). An error will result if it can't do this - for example if
   the result is empty - so you may wish to just stick to 'text' as the
   column type if you think your data has any problems.
  </para>
  <para>
   The select statement doesn't need to use * alone - it can reference the
   columns by name or join them to other tables. The function produces a
   virtual table with which you can perform any operation you wish (e.g.
   aggregation, joining, sorting etc). So we could also have:
  </para>

  <programlisting>
SELECT t.title, p.fullname, p.email 
FROM xpath_table('article_id','article_xml','articles',
            '/article/title|/article/author/@id',
            'xpath_string(article_xml,''/article/@date'') > ''2003-03-20'' ')
            AS t(article_id integer, title text, author_id integer), 
     tblPeopleInfo AS p 
WHERE t.author_id = p.person_id;
  </programlisting>

  <para>
   as a more complicated example. Of course, you could wrap all
   of this in a view for convenience.
  </para>
  <sect3>
   <title>Multivalued results</title>
   <para>
    The xpath_table function assumes that the results of each XPath query
    might be multi-valued, so the number of rows returned by the function
    may not be the same as the number of input documents. The first row
    returned contains the first result from each query, the second row the
    second result from each query. If one of the queries has fewer values
    than the others, NULLs will be returned instead.
   </para>
   <para>
    In some cases, a user will know that a given XPath query will return
    only a single result (perhaps a unique document identifier) - if used
    alongside an XPath query returning multiple results, the single-valued
    result will appear only on the first row of the result. The solution
    to this is to use the key field as part of a join against a simpler
    XPath query. As an example:
   </para>
   
   <para>
    <literal>
    CREATE TABLE test
    (
      id int4 NOT NULL,
      xml text,
      CONSTRAINT pk PRIMARY KEY (id)
    ) 
    WITHOUT OIDS;
    
    INSERT INTO test VALUES (1, '&lt;doc num="C1">
    &lt;line num="L1">&lt;a>1&lt;/a>&lt;b>2&lt;/b>&lt;c>3&lt;/c>&lt;/line>
    &lt;line num="L2">&lt;a>11&lt;/a>&lt;b>22&lt;/b>&lt;c>33&lt;/c>&lt;/line>
    &lt;/doc>');
    
    INSERT INTO test VALUES (2, '&lt;doc num="C2">
    &lt;line num="L1">&lt;a>111&lt;/a>&lt;b>222&lt;/b>&lt;c>333&lt;/c>&lt;/line>
    &lt;line num="L2">&lt;a>111&lt;/a>&lt;b>222&lt;/b>&lt;c>333&lt;/c>&lt;/line>
    &lt;/doc>');
    </literal>
   </para>
  </sect3>
   
  <sect3>
   <title>The query</title>
   
   <programlisting>
   SELECT * FROM  xpath_table('id','xml','test', 
   '/doc/@num|/doc/line/@num|/doc/line/a|/doc/line/b|/doc/line/c','1=1') 
   AS t(id int4, doc_num varchar(10), line_num varchar(10), val1 int4, 
   val2 int4, val3 int4)
   WHERE id = 1 ORDER BY doc_num, line_num
   </programlisting>
   
   <para>
   Gives the result:
   </para>
   
   <programlisting>
    id | doc_num | line_num | val1 | val2 | val3
   ----+---------+----------+------+------+------
     1 | C1      | L1       |    1 |    2 |    3
     1 |         | L2       |   11 |   22 |   33
   </programlisting>
   
   <para>
   To get doc_num on every line, the solution is to use two invocations
   of xpath_table and join the results:
   </para>
   
   <programlisting>
   SELECT t.*,i.doc_num FROM 
     xpath_table('id','xml','test',
      '/doc/line/@num|/doc/line/a|/doc/line/b|/doc/line/c','1=1') 
           AS t(id int4, line_num varchar(10), val1 int4, val2 int4, val3 int4),
     xpath_table('id','xml','test','/doc/@num','1=1') 
           AS i(id int4, doc_num varchar(10))
   WHERE i.id=t.id AND i.id=1
   ORDER BY doc_num, line_num;
   </programlisting>
   
   <para>
   which gives the desired result:
   </para>
   
   <programlisting>
    id | line_num | val1 | val2 | val3 | doc_num
   ----+----------+------+------+------+---------
     1 | L1       |    1 |    2 |    3 | C1
     1 | L2       |   11 |   22 |   33 | C1
   (2 rows)
   </programlisting>
  </sect3>
 </sect2>
   

 <sect2>
  <title>XSLT functions</title>
  <para>
   The following functions are available if libxslt is installed (this is
   not currently detected automatically, so you will have to amend the
   Makefile)
  </para>

  <sect3>
   <title><literal>xslt_process</literal></title>
   <programlisting>
    xslt_process(document,stylesheet,paramlist) RETURNS text
   </programlisting>

   <para>
    This function appplies the XSL stylesheet to the document and returns
    the transformed result. The paramlist is a list of parameter
    assignments to be used in the transformation, specified in the form
    'a=1,b=2'. Note that this is also proof-of-concept code and the
    parameter parsing is very simple-minded (e.g. parameter values cannot
    contain commas!)
   </para>
   <para>
    Also note that if either the document or stylesheet values do not
    begin with a < then they will be treated as URLs and libxslt will
    fetch them. It thus follows that you can use xslt_process as a means
    to fetch the contents of URLs - you should be aware of the security
    implications of this.
   </para> 
   <para>
    There is also a two-parameter version of xslt_process which does not
    pass any parameters to the transformation.
   </para>
  </sect3>
 </sect2>

 <sect2>
  <title>Credits</title>
  <para>
   Development of this module was sponsored by Torchbox Ltd. (www.torchbox.com)
   It has the same BSD licence as PostgreSQL.
  </para>
  <para>
   This version of the XML functions provides both XPath querying and
   XSLT functionality. There is also a new table function which allows
   the straightforward return of multiple XML results. Note that the current code
   doesn't take any particular care over character sets - this is
   something that should be fixed at some point!
  </para>
  <para>
   If you have any comments or suggestions, please do contact me at
   <email>jgray@azuli.co.uk.</email> Unfortunately, this isn't my main job, so 
   I can't guarantee a rapid response to your query!
  </para>
 </sect2>
</sect1>

