XML-handling functions for PostgreSQL
=====================================

Development of this module was sponsored by Torchbox Ltd. (www.torchbox.com)
It has the same BSD licence as PostgreSQL.

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!

Installation
------------

The current build process will only work if the files are in
contrib/xml2 in a PostgreSQL 7.3 or later source tree which has been
configured and built (If you alter the subdir value in the Makefile
you can place it in a different directory in a PostgreSQL tree).

Before you begin, just check the Makefile, and then just 'make' and
'make install'.

This code requires libxml to be previously installed.

Description of functions
------------------------

The first set of functions are straightforward XML parsing and XPath queries:

xml_valid(document) RETURNS bool

This parses the document text in its parameter and returns true if the
document is well-formed XML.

xpath_string(document,query) RETURNS text
xpath_number(document,query) RETURNS float4
xpath_bool(document,query) RETURNS bool

These functions evaluate the XPath query on the supplied document, and
cast the result to the specified type.


xpath_nodeset(document,query,toptag,itemtag) RETURNS text

This evaluates query on document and wraps the result in XML tags. If
the result is multivalued, the output will look like:

<toptag>
<itemtag>Value 1 which could be an XML fragment</itemtag>
<itemtag>Value 2....</itemtag>
</toptag>

If either toptag or itemtag is an empty string, the relevant tag is omitted.
There are also wrapper functions for this operation:

xpath_nodeset(document,query) RETURNS text omits both tags.
xpath_nodeset(document,query,itemtag) RETURNS text omits toptag.


xpath_list(document,query,seperator) RETURNS text

This function returns multiple values seperated by the specified
seperator, e.g. Value 1,Value 2,Value 3 if seperator=','.

xpath_list(document,query) RETURNS text

This is a wrapper for the above function that uses ',' as the seperator.


xpath_table
-----------

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.

The function itself takes 5 arguments, all text.

xpath_table(key,document,relation,xpaths,criteria)

key - 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).

document - the name of the field containing the XML document

relation - the name of the table or view containing the documents

xpaths - multiple xpath expressions separated by |

criteria - 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.

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

SELECT <key>,<document> FROM <relation> WHERE <criteria>

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.

Using the function

The function has to be used in a FROM expression. This gives the following
form:

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);

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.

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.

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:

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;

as a more complicated example. Of course, you could wrap all
of this in a view for convenience.

Multivalued results

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.

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:


CREATE TABLE test
(
  id int4 NOT NULL,
  xml text,
  CONSTRAINT pk PRIMARY KEY (id)
) 
WITHOUT OIDS;

INSERT INTO test VALUES (1, '<doc num="C1">
<line num="L1"><a>1</a><b>2</b><c>3</c></line>
<line num="L2"><a>11</a><b>22</b><c>33</c></line>
</doc>');

INSERT INTO test VALUES (2, '<doc num="C2">
<line num="L1"><a>111</a><b>222</b><c>333</c></line>
<line num="L2"><a>111</a><b>222</b><c>333</c></line>
</doc>');


The query:

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


Gives the result:

 id | doc_num | line_num | val1 | val2 | val3
----+---------+----------+------+------+------
  1 | C1      | L1       |    1 |    2 |    3
  1 |         | L2       |   11 |   22 |   33

To get doc_num on every line, the solution is to use two invocations
of xpath_table and join the results:

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;

which gives the desired result:

 id | line_num | val1 | val2 | val3 | doc_num
----+----------+------+------+------+---------
  1 | L1       |    1 |    2 |    3 | C1
  1 | L2       |   11 |   22 |   33 | C1
(2 rows)



XSLT functions
--------------

The following functions are available if libxslt is installed (this is
not currently detected automatically, so you will have to amend the
Makefile)

xslt_process(document,stylesheet,paramlist) RETURNS text

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!)

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.

There is also a two-parameter version of xslt_process which does not
pass any parameters to the transformation.


Feedback
--------

If you have any comments or suggestions, please do contact me at
jgray@azuli.co.uk. Unfortunately, this isn't my main job, so I can't
guarantee a rapid response to your query!
