Re: define views using xquery

Messageone of the actual difficulties is like that:

there is a XML document (spj.xml):
<db>

  <project jno="j001">

    <supplier sno="s001">

        <part pno="p001">

           <price> 100</price>

        </part>

    </supplier>

    <supplier sno="s002">

        <part pno="p001">

            <price> 110</price>

        </part>

    </supplier>

  </project>

</db>

 

we want to define a view which selects such project, supplier and part that their sub element price is greater than 100.  the view definition may be:



define view test as

For      $j In document("spj.xml")//project[//price >100]

             Return 

                      <project jno = {$j/@jno}>

                        { For $s In $j/supplier[//price >100]

                           Return

                                  <supplier sno = {$s/@sno}> 

                                    { For $p In $s/part[price > 100]

                                       Return                          

                                         <part pno = {$p/@pno}> 

                                               {$p/price} 

                                         </part>

                                     } 

                                  </supplier>

                        } 

                     </project>



we can see that for such a simple view, we need to repeat the predicate "price >100" three times, which will lead to a very low performance of query processing, because we need to search the tree three times. the view definition is not only complex and wordy, but also low-performance.

Is there any other way to write the query?



as a view definition, one of favorite feature is that it is declarative, so that we don't need to write a very complex definition.



can we just use a more simple way to write view, can XML Algebra provide any help?



the view is just to put a predicate to filter some elements. if we define a view that change some structures, the view definition will be more complex.



until now, we do not consider the problem of view updatability and the combination of XML-Schema and XQuery.



Best regards,



  ----- Original Message ----- 
  From: Peter Fankhauser 
  To: 'Chen Ya Bing' ; www-ql@w3.org 
  Sent: Thursday, February 21, 2002 9:40 PM
  Subject: RE: define views using xquery


  As a starter from the scientific side you may check out
  some recent papers by Serge Abiteboul and others (one of
  Serge's favorite themes is views in all disguises).

  I'd be also be interested in the actual difficulties that you
  encountered when trying views with XQuery. In principle
  one can regard a view as a "stored" query which can be
  composed with other queries. As one of XQuery's virtues
  is compositionality, another is closure, so I wonder where exactly
  the difficulty lies.

  (1) Is it the combination of XML-Schema and XQuery that
  you (presumably) need.

  (2) Is it updatability of views that concern you?

  (3) Is it that your notion of "simple view" doesn't translate
  to an XQuery notion of a "simple query"?

  (4) Which sort of views do you want to define?

  Have you got any concrete examples, usecases, requirements?
  Where exactly do you encounter inefficiency?

  Best regards,

  Peter


    -----Original Message-----
    From: www-ql-request@w3.org [mailto:www-ql-request@w3.org] On Behalf Of Chen Ya Bing
    Sent: Donnerstag, 21. Februar 2002 10:02
    To: www-ql@w3.org
    Subject: define views using xquery


    I'd like to use XQuery as a view definition language to define XML views. but I find that it is difficult to use it as the view definition language, because a simple view definition have to lead to a complex XQuery. even more, it may be inefficient. comparing with view definition in SQL, it is inconvenient.

    can anybody give me advices of how to utilize XQuery to define XML view more efficiently?
    and how will the XQuery WD support the view facility.

    Best regards,
    Chen