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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'. By default, this module requires both libxml2 and libxslt to be installed on your system. If you do not have libxslt or do not want to use XSLT functions, you must edit the Makefile to not build the XSLT functions, as directed in its comments; and edit pgxml.sql.in to remove the XSLT function declarations, as directed in its comments. Description of functions ------------------------ The first set of functions are straightforward XML parsing and XPath queries: xml_is_well_formed(document) RETURNS 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 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.) 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 email@example.com. Unfortunately, this isn't my main job, so I can't guarantee a rapid response to your query!