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parser interface code. It now uses libxml2 instead of expat (though I've left the old code in the tarball). This means *proper* XPath support, and the provided function allows you to wrap your result set in XML tags to produce a new XML document. John Gray
79 lines
2.5 KiB
Plaintext
79 lines
2.5 KiB
Plaintext
PGXML TODO List
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===============
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Some of these items still require much more thought! Since the first
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release, the XPath support has improved (because I'm no longer using a
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homemade algorithm!).
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1. Performance considerations
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At present each document is parsed to produce the DOM tree on every query.
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Pros:
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Easy
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No persistent memory or storage allocation for parsed trees
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(libxml docs suggest representation of a document might
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be 4 times the size of the text)
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Cons:
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Slow/ CPU intensive to parse.
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Makes it difficult for PLs to apply libxml manipulations to create
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new documents or amend existing ones.
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2. XQuery
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I'm not sure if the addition of XQuery would be best as a function or
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as a new front-end parser. This is one to think about, but with a
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decent implementation of XPath, one of the prerequisites is covered.
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3. DOM Interfaces
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Expose more aspects of the DOM to user functions/ PLs. This would
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allow a procedure in a PL to run some queries and then use exposed
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interfaces to libxml to create an XML document out of the query
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results. I accept the argument that this might be more properly
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performed on the client side.
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4. Returning sets of documents from XPath queries.
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Although the current implementation allows you to amalgamate the
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returned results into a single document, it's quite possible that
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you'd like to use the returned set of nodes as a source for FROM.
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Is there a good way to optimise/index the results of certain XPath
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operations to make them faster?:
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select docid, pgxml_xpath(document,'//site/location/text()','','') as location
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where pgxml_xpath(document,'//site/name/text()','','') = 'Church Farm';
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and with multiple element occurences in a document?
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select d.docid, pgxml_xpath(d.document,'//site/location/text()','','')
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from docstore d,
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pgxml_xpaths('docstore','document','//feature/type/text()','docid') ft
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where ft.key = d.docid and ft.value ='Limekiln';
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pgxml_xpaths params are relname, attrname, xpath, returnkey. It would
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return a set of two-element tuples (key,value) consisting of the value of
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returnkey, and the cdata value of the xpath. The XML document would be
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defined by relname and attrname.
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The pgxml_xpaths function could be the basis of a functional index,
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which could speed up the above query very substantially, working
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through the normal query planner mechanism.
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5. Return type support.
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Better support for returning e.g. numeric or boolean values. I need to
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get to grips with the returned data from libxml first.
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John Gray <jgray@azuli.co.uk> 16 August 2001
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