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Re: XQuery and relational databases...

From: Jim Melton <jim.melton@acm.org>
Date: Fri, 28 Feb 2003 11:29:17 -0700
Message-Id: <4.3.2.7.2.20030228111535.052c00b0@gmstimap.oraclecorp.com>
To: Michael Burbidge <mburbidg@adobe.com>
Cc: www-ql@w3.org

Michael,

I will not pretend to give an *official* Query WG response, so please 
recognize that this note contains only my perception of the reasons.

One of several major goals adopted when XQuery was designed was that it be 
able to deal with queries on relational-like data.  Several major vendors 
of XML systems and of relational systems explicitly want to be able to use 
XQuery to query relational data that is "published" in an XML form out of a 
relational database.

However, I don't think that this was the primary motivation for the 
creation of the FLWOR expression.  One very important attribute of SQL is 
that it is primarily declarative in nature, not procedural.  By that, I 
mean that SQL allows a user to specify what characteristics the desired 
results have, but not the algorithms by which those results are 
obtained.  That attribute is extremely important in several ways, but most 
importantly in allowing query optimizers flexibility in putting together 
"query plans" that operate specific queries differently in different 
environments.  XQuery's design is also based on the 
declarative-instead-of-procedural-language philosophy, specifically for the 
same reasons that SQL was based on that philosophy.

That characteristic by itself is not enough to determine whether or not 
FLWOR expressions can be translated to SQL or SQL to FLWOR 
expressions.  However, I have it on extremely good authority ;^} that it is 
eminently feasible to translate a great fraction, possibly 100%, of XQuery 
expressions into SQL.  Whether or not the reverse translation is feasible, 
I don't have concrete information.  However, it would surprise me if it 
were not possible to do so in some large percentage of cases.

And, yes, you may confidently expect that relational database vendors will 
implement XQuery processors that directly integrate with their 
databases.  Whether this is done "in an efficient manner" will certainly be 
in the eye of the beholder.  Some will undoubtedly put their XQuery 
processors deeply embedded into their relational engines, while others will 
provide XQuery as a layer on top of the engine, while others will probably 
provide XQuery in middleware...all based on the individual vendor's 
perception of its marketplace's requirements.

Hope this helps,
    Jim

At 07:56 2003-02-28 -0800 Friday, Michael Burbidge wrote:

>The XQuery specifications alludes to the fact that XQuery was designed in 
>such a way as to be able express queries across various kinds of data, 
>including relational databases.
>
>Does this mean that the FLWOR expression was designed to support
>queries that can be performed on relational databases? Was it designed to 
>be roughly equivalent to SQL? Given that a relational database has some 
>mechanism for exposing XML views on relational tables, is it relatively 
>straight forward to convert an FLWOR expressions into SQL? Do we expect 
>that relational database vendors will implement XQuery processors that 
>directly integrate with their databases in an efficient manner?
>
>Thanks,
>Michael-
>

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Jim Melton --- Editor of ISO/IEC 9075-* (SQL)     Phone: +1.801.942.0144
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Received on Friday, 28 February 2003 13:32:48 GMT

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