[xquery-talk] XQuery error codes and rewriting

Daniela Florescu dflorescu at mac.com
Thu Feb 2 14:16:57 PST 2006

>  it would be nice if either those controls were standard

This would imply that the optimizations we do are standard,
and of course that's not a viable solution.

> The ability to selectively or globally disable these optimizations may
> also be helpful (especially in distinguishing bugs vs. features)

I am sure that implementations will allow users to control
the compromise between correctness and performance,
and eventually give compilation and/or execution feedback.

Just put some pressure on your favorite vendor....:-)

Best regards,

>  but
> that's likely too difficult to apply generally to XQuery
> implementations.
> Jeff.
> -----Original Message-----
> From: talk-bounces at xquery.com [mailto:talk-bounces at xquery.com] On 
> Behalf
> Of Michael Rys
> Sent: Thursday, February 02, 2006 1:40 PM
> To: Daniela Florescu; Michael Kay
> Cc: Peter Coppens; talk at xquery.com; David Carlisle; John Snelson
> Subject: RE: [xquery-talk] XQuery error codes and rewriting
> Importance: Low
> Dana is correct. Except for that the latest optimizing hardware
> processors actually do stuff that make C and Java programs result in
> wrong results according to the language semantics....
> Michael
>> -----Original Message-----
>> From: talk-bounces at xquery.com [mailto:talk-bounces at xquery.com] On
> Behalf
>> Of Daniela Florescu
>> Sent: Thursday, February 02, 2006 1:21 PM
>> To: Michael Kay
>> Cc: talk at xquery.com; 'Peter Coppens'; 'David Carlisle'; 'John Snelson'
>> Subject: Re: [xquery-talk] XQuery error codes and rewriting
>> Michael,
>> this has nothing to do with SQL and databases per se. It has
>> something to do with do optimizations.
>> The kind of optimizations and rewritings a database does for
>> its declarative languages (and that make this entire industry so
>> useful) are inherently in tension with a 100% specified semantics
>> of such languages.
>> Imperative programming languages like Java and C have a
>> very clear semantics, tightly controlled, but as a result
>>   they are limited in terms of the rewritings and optimization
>> they can do while being strict about the semantics.
>> In XQuery we defined the semantics in such a way that it leaves
>> the door open for some optimizations that we knew in advance
>> that will be useful (like shortcircuiting quantifiers).
>> But there are hundreds of other rewritings that people will invent
>> for XQuery over time, that we didn't anticipate when we wrote the
>> semantics.
>> Such implementations will not be conformant at 100%. But that's fine.
>> If they give users an order of magnitude better performance they'll
>> buy it despite that.
>> The tension is between determinist semantics and optimization,
>> and I am not aware of a good answer for this tension.
>> Best regards,
>> Dana
>> On Feb 2, 2006, at 12:39 PM, Michael Kay wrote:
>>>> Was it never considered to remove that paragraph altogether?
>>>> I am sure that in that case some implementations will never be able
> to
>>>> fully comply, but at least to me that seems preferable over what
> you
>>>> have now.
>>> There are one or two people on the working group who incline to this
>>> kind of
>>> position, that when it comes to the complicated edge cases, it
> doesn't
>>> matter what we say because implementors will do their own thing
>>> anyway. This
>>> kind of thinking tends to come, I think, from people from the
> database
>>> community where the expectations that products will conform 100% to
>>> SQL or
>>> any other standard are historically quite low. On the whole, though,
>>> the
>>> level of conformance of products in the XML space is much better,
> and
>>> if you
>>> look at the core specs such as XML itself and XSLT, even the most
> minor
>>> infringements of the spec give a product a lot of flak. For many of
> us
>>> therefore, it's very important that the spec spells out exactly what
> is
>>> allowed and what isn't, even if this sometimes means being more
>>> liberal than
>>> one would like for 100% interoperability.
>>> Michael Kay
>>> http://www.saxonica.com/
>>> _______________________________________________
>>> talk at xquery.com
>>> http://xquery.com/mailman/listinfo/talk
>> _______________________________________________
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