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About XQuantum

XQuantum is an XML database that stores information and data as XML rather than in relational tables. The system fully implements the XQuery standard developed by the World Wide Web Consortium, W3C.

Language Standard
XQuantum is based on the January 23, 2007 W3C standard, XQuery 1.0: An XML Query Language and the May 1, 2006 W3C draft standard, XQuery 1.0 and XPath 2.0 Full-Text.

Query Engine
Query optimization, plan generation, and indexing can improve performance thousands of times depending on the query.  These techniques become essential whenever the size of the XML data is greater than roughly 20 megabytes. 

XQuantum uses cost-based query plan generation, which uses statistics about the XML data to choose the best search algorithm.  The diagram below shows the various query processing steps:


Query processing steps used by XQuantum

During processing, the query is converted into an expression belonging to an abstract algebra.  The expression is then rewritten into a normal form using the algebra's identity rules.  Next, a query plan is generated by computing the cost of using different algorithms and selecting the best one.

The optimized plan is then executed, producing a ground expression stream; (a ground expression is a constant expression with no functions or variables).  The ground expression is then serialized or converted into an XML stream.

The Type System
XQuery is a statically typed language, which uses type information to statically check queries for correctness.  XQuantum generalizes XQuery's sequence type syntax to include full regular expression types.  The extended type system makes working with XML data types much simpler.

XQuantum types XML content using optional type definitions stored in external type modules called .rtd files.  A type module is the counterpart of a query module and defines the data type for a single XML file.  During query evaluation the validator applies this type information to the XML content, creating a typed ground expression stream.

References
Information on the XQuery standard can be found at the W3C web site,
www.w3.org/XML/Query.  The XQuery 1.0: A Query Language for XML document describes the XQuery language, and the XQuery 1.0 and XPath 2.0 Full-Text document describes the XQuery full-text extensions.


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