The IVOA in 2006: Assessment and Future Roadmap - Registries

[Brian Thomas]

On Thursday 08 June 2006 14:30, Tony Linde wrote:
> And the prime difficulty is not mapping from the xml schema to the
> relational schema but in mapping an XQuery statement to some underlying
> database language: SQL would not be enough, you'd need the stored procedure
> language as well and even that may not provide everything XQuery does, even
> if you could code the mapping.

	The mapping is not that bad, and its simple actually... it basically means 
	mapping column values into a special node (say 'value') in the XML document
	which may be treated specially and is logically thought of as holding lists of
	values. 

	To explain with an example, consider that you have some meta-data stored in the 
	XML document itself, and some (meta-) data stored in the RDBMS, as needed. 
	The XML looks like:

	<mydoc>
	    <elementWithRDBMSvalues name="thisField">
                       <metaDataNode>meta-data value</metaDataNode>
	         <orml:value dbId="myDb" dbTable="myTable" dbCol="myColumn"/>
	    </elementWithRDBMSvalues>
	    ....
	</mydoc>

	Now, the XQuery to pull back (all of ) the data in that node :

declare namespace a="urn:mynamespace";

<result> {
    for $field in /a:document//elementWithRDBMSvalues
     return <field> { $field } </c:field>
} </result>

	returns all the possible values in the RDBMS, namely "10 32 43 44 1882" and 
	would look like:
	
	<field name="thisField">
                     <metaDataNode>meta-data value</metaDataNode>
	       <a:value>10 32 43 44 1882</a:value>
	</field>
	
	By adding a "select" function to XQuery, one can get select-like functionality
	on the XQuery nodes which hold the relational values, e.g.


declare namespace c="urn:mynamespace";
declare namespace orml="urn:ormlmappedvalues";
<result> {
for $field in //a:field
  where $field[@name="thisField"]
  return select { $field where //a:field/orml:value  < 30.0 }
} </result>

	Then returns:
	
	<field name="thisField">
                     <metaDataNode>meta-data value</metaDataNode>
	       <a:value>32 43 44 1882</a:value>
	</field>

	
	Having a native XQuery is nice, but doesn't do the full job: you either have to store
	your data in terms of the object model of the VO, something which most of us would
	rather avoid (re-mapping your DB to VO model, yuk) or you have to remap the XQuery
	in terms of your local schema (double yuk). The better solution is to lay an object
	layer on top of the RDBMS which implements a mapping from a shared, community object
	model on top of the local relational schema.

	=brian