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RE: Parallel execution: functions, XPath, XQuery,

Hi Ken

Due to the side-effect free nature of the FLWR expressions in XQuery
(and the functional semantics in XPath), I see no problem of being able
to parallelize XQuery expressions (assuming that UDFs are written
side-effect free as well).

Do you have any specific point in the functionality that makes you
believe otherwise?

Best regards

> -----Original Message-----
> From: Ken North [mailto:ken_north@compuserve.com] 
> Sent: Monday, May 21, 2001 10:02 AM
> To: xml-dev@lists.xml.org; W3C Query Comments; XPath Comments
> Subject: Parallel execution: functions, XPath, XQuery,
> A review of the current working documents for XQuery and 
> XPath 2.0 requirements raises some questions about building a 
> search engine that can be deployed over a parallel processing 
> architecture.
> Because SQL query optimizers can "parallelize" operations, 
> SQL engines can exploit parallel architectures and use 
> multiple processors for query evaluation and execution. 
> Clearly we want XML processing to be able to exploit 
> massively parallel processing, clusters, and other parallel 
> architectures, but there are issues:
> 1. SQL is set-oriented so it's possible to partition data across
> (processing) nodes, partition queries, and combine results. 
> An SQL query doesn't define a navigation path for getting to 
> data. The query just specifies what's wanted and a query 
> optimizer determines the most efficient means of execution.
> So issue 1 is how to partition document collections for 
> optimal parallel execution with XPath and XQuery. I've not 
> found any published research on parallel evaluation or 
> parallel execution with XPath. Anyone have a URL?
> 2. The XPath 2.0 requirements document identifies the need 
> for additional string and aggregate functions, but doesn't 
> define a requirement for being able to invoke user-defined 
> functions (UDFs). XQuery supports XPath functions but it also 
> supports UDFs. A future version will support UDFs implemented 
> in programming languages.
> So issue 2 is the future direction of XPath, particularly 
> functions and extensibility.
> 3. If you are using an SQL products such as Oracle 8i and IBM 
> DB2, you can add UDFs for processing XML data by implementing 
> them in Java, C/C++ and deploying them as a database plug-in. 
> For example, by implementing FOO() in Java, I can use SELECT 
> FOO() in SQL queries. It's possible to use FOO() in parallel 
> queries. DB2, for example, provides an ALLOW PARALLEL option 
> when I register my function classes with the database. 
> Therefore, SQL queries can still be "parallelized" if they 
> include my Java, C/C++ UDFs.
> So issue 3 is how to express to XPath or XQuery details such 
> as whether to use a single instance of the Java class per 
> query or allow parallel use of an UDF.
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