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119 lines
3.4 KiB
Plaintext
119 lines
3.4 KiB
Plaintext
This package contains some simple routines for manipulating XML
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documents stored in PostgreSQL. This is a work-in-progress and
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somewhat basic at the moment (see the file TODO for some outline of
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what remains to be done).
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At present, two modules (based on different XML handling libraries)
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are provided.
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Prerequisite:
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pgxml.c:
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expat parser 1.95.0 or newer (http://expat.sourceforge.net)
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or
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pgxml_dom.c:
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libxml2 (http://xmlsoft.org)
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The libxml2 version provides more complete XPath functionality, and
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seems like a good way to go. I've left the old versions in there for
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comparison.
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Compiling and loading:
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----------------------
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The Makefile only builds the libxml2 version.
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To compile, just type make.
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Then you can use psql to load the two function definitions:
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\i pgxml_dom.sql
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Function documentation and usage:
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---------------------------------
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pgxml_parse(text) returns bool
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parses the provided text and returns true or false if it is
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well-formed or not. It returns NULL if the parser couldn't be
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created for any reason.
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pgxml_xpath (XQuery functions) - differs between the versions:
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pgxml.c (expat version) has:
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pgxml_xpath(text doc, text xpath, int n) returns text
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parses doc and returns the cdata of the nth occurence of
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the "simple path" entry.
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However, the remainder of this document will cover the pgxml_dom.c version.
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pgxml_xpath(text doc, text xpath, text toptag, text septag) returns text
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evaluates xpath on doc, and returns the result wrapped in
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<toptag>...</toptag> and each result node wrapped in
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<septag></septag>. toptag and septag may be empty strings, in which
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case the respective tag will be omitted.
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Example:
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Given a table docstore:
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Attribute | Type | Modifier
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-----------+---------+----------
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docid | integer |
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document | text |
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containing documents such as (these are archaeological site
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descriptions, in case anyone is wondering):
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<?XML version="1.0"?>
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<site provider="Foundations" sitecode="ak97" version="1">
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<name>Church Farm, Ashton Keynes</name>
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<invtype>watching brief</invtype>
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<location scheme="osgb">SU04209424</location>
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</site>
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one can type:
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select docid,
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pgxml_xpath(document,'//site/name/text()','','') as sitename,
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pgxml_xpath(document,'//site/location/text()','','') as location
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from docstore;
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and get as output:
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docid | sitename | location
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-------+--------------------------------------+------------
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1 | Church Farm, Ashton Keynes | SU04209424
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2 | Glebe Farm, Long Itchington | SP41506500
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3 | The Bungalow, Thames Lane, Cricklade | SU10229362
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(3 rows)
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or, to illustrate the use of the extra tags:
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select docid as id,
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pgxml_xpath(document,'//find/type/text()','set','findtype')
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from docstore;
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id | pgxml_xpath
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----+-------------------------------------------------------------------------
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1 | <set></set>
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2 | <set><findtype>Urn</findtype></set>
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3 | <set><findtype>Pottery</findtype><findtype>Animal bone</findtype></set>
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(3 rows)
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Which produces a new, well-formed document. Note that document 1 had
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no matching instances, so the set returned contains no
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elements. document 2 has 1 matching element and document 3 has 2.
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This is just scratching the surface because XPath allows all sorts of
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operations.
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Note: I've only implemented the return of nodeset and string values so
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far. This covers (I think) many types of queries, however.
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John Gray <jgray@azuli.co.uk> 16 August 2001
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