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F.42. xml2
The xml2 module provides XPath querying and XSLT functionality.
F.42.1. Deprecation notice
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, 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.
F.42.2. Description of functions
Table F-31 shows the functions provided by this module. These functions provide straightforward XML parsing and XPath queries. All arguments are of type text, so for brevity that is not shown.
Table F-31. Functions
| Function | Returns | Description |
|---|---|---|
xml_is_well_formed(document)
|
bool | 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 |
xpath_string(document, query)
|
text | These functions evaluate the XPath query on the supplied document, and cast the result to the specified type. |
xpath_number(document, query)
|
float4 | |
xpath_bool(document, query)
|
bool | |
xpath_nodeset(document, query, toptag, itemtag)
|
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. |
xpath_nodeset(document, query)
|
text | Like |
xpath_nodeset(document, query, itemtag)
|
text | Like |
xpath_list(document, query, separator)
|
text | This function returns multiple values separated by the specified separator, for example Value 1,Value 2,Value 3 if separator is ,. |
xpath_list(document, query)
|
text | This is a wrapper for the above function that uses , as the separator. |
F.42.3. xpath_table
xpath_table(text key, text document, text relation, text xpaths, text criteria) returns setof record
xpath_table is a table function that 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 result set
can readily be used in joins. The parameters are described in
Table F-32.
Table F-32. xpath_table Parameters
| Parameter | Description |
|---|---|
| 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 | one or more XPath expressions, separated by | |
| criteria | the contents of the WHERE clause. This cannot be omitted, so use true or 1=1 if you want to process all the rows in the relation |
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.
The function has to be used in a FROM expression, with an AS clause to specify the output columns; for example
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 output table. The first is the "key" field and the rest correspond to the XPath queries. 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.
Notice that this example defines the page_count result column as 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 calling SELECT statement doesn't necessarily have be be just SELECT * — it can reference the output 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.
F.42.3.1. 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, null values 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 int PRIMARY KEY,
xml text
);
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>');
SELECT * FROM
xpath_table('id','xml','test',
'/doc/@num|/doc/line/@num|/doc/line/a|/doc/line/b|/doc/line/c',
'true')
AS t(id int, doc_num varchar(10), line_num varchar(10), val1 int, val2 int, val3 int)
WHERE id = 1 ORDER BY doc_num, line_num
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',
'true')
AS t(id int, line_num varchar(10), val1 int, val2 int, val3 int),
xpath_table('id', 'xml', 'test', '/doc/@num', 'true')
AS i(id int, doc_num varchar(10))
WHERE i.id=t.id AND i.id=1
ORDER BY doc_num, line_num;
id | line_num | val1 | val2 | val3 | doc_num
----+----------+------+------+------+---------
1 | L1 | 1 | 2 | 3 | C1
1 | L2 | 11 | 22 | 33 | C1
(2 rows)
F.42.4. XSLT functions
The following functions are available if libxslt is installed:
F.42.4.1. xslt_process
xslt_process(text document, text stylesheet, text paramlist) returns text
This function applies 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 the parameter parsing is very simple-minded: 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 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.
F.42.5. Author
John Gray <jgray@azuli.co.uk>
Development of this module was sponsored by Torchbox Ltd. (www.torchbox.com). It has the same BSD licence as PostgreSQL.