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84 lines
2.9 KiB
Plaintext
84 lines
2.9 KiB
Plaintext
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PGXML TODO List
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===============
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Some of these items still require much more thought! The data model
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for XML documents and the parsing model of expat don't really fit so
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well with a standard SQL model.
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1. Generalised XML parsing support
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Allow a user to specify handlers (in any PL) to be used by the parser.
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This must permit distinct sets of parser settings -user may want some
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documents in a database to parsed with one set of handlers, others
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with a different set.
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i.e. the pgxml_parse function would take as parameters (document,
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parsername) where parsername was the identifier for a collection of
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handler etc. settings.
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"Stub" handlers in the pgxml code would invoke the functions through
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the standard fmgr interface. The parser interface would define the
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prototype for these functions. How does the handler function know
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which document/context has resulted it in being called?
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Mechanism for defining collection of parser settings (in a table? -but
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maybe copied for efficiency into a structure when first required by a
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query?)
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2. Support for other parsers
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Expat may not be the best choice as a parser because a new parser
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instance is needed for each document i.e. all the handlers must be set
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again for each document. Another parser may have a more efficient way
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of parsing a set of documents identically.
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3. XPath support
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Proper XPath support. I really need to sit down and plough
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through the specification...
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The very simple text comparison system currently used is too
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basic. Need to convert the path to an ordered list of nodes. Each node
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is an element qualifier, and may have a list of attribute
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qualifications attached. This probably requires lexx/yacc combination.
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(James Clark has written a yacc grammar for XPath). Not all the
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features of XPath are necessarily relevant.
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An option to return subdocuments (i.e. subelements AND cdata, not just
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cdata). This should maybe be the default.
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4. Multiple occurences of elements.
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This section is all very sketchy, and has various weaknesses.
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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',1) as location
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where pgxml_xpath(document,'/site/name',1) = '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',1)
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from docstore d,
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pgxml_xpaths('docstore','document','feature/type','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. Syntax above is fragile
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through using names rather than OID.
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John Gray <jgray@azuli.co.uk>
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