[xquery-talk] Function for determining one XPath as subset of another

Pavel Velikhov pavel.velikhov at gmail.com
Wed Jan 27 02:24:12 PST 2016


> 
> Indeed, the expressions should terminate.

Since you can include a function call in your path expression, and you can include your own custom function, there is
no guarantee that the expression will terminate.

I.e. you need to take a subset of possible path expression already, in general case the problem is undecidable.

> 
> 2016-01-27 11:11 GMT+01:00 Pavel Velikhov <pavel.velikhov at gmail.com <mailto:pavel.velikhov at gmail.com>>:
> 
>> 
>> Or simplified: is the set selected by p1 equal to the set selected by p2?
> 
> If we allow p1 and p2 to be arbitrary XQuery path expressions, then its undecidable.
> I can reduce the halting problem of Turing machines to this test.
> 
>> 
>> 2016-01-27 11:09 GMT+01:00 W.S. Hager <wshager at gmail.com <mailto:wshager at gmail.com>>:
>> Isn't the constraint in this case the test: is p2 a subset of p1?
>> 
>> 2016-01-27 11:04 GMT+01:00 Pavel Velikhov <pavel.velikhov at gmail.com <mailto:pavel.velikhov at gmail.com>>:
>> 
>> > On 27 Jan 2016, at 12:54, W.S. Hager <wshager at gmail.com <mailto:wshager at gmail.com>> wrote:
>> >
>> > Can't we formally proof something as obvious Adam's case?
>> 
>> In Adam’s case we want to test whether path expression p1 subsumes path expression p2.
>> If we don’t put any conditions on p1 and p2, the problem is undecidable: p1 and p2 may include
>> function calls, so the expressive power of p1 and p2 are that of a Turing Machine.
>> 
>> 
>> 
>> -- 
>> W.S. Hager
>> Lagua Web Solutions
>> http://lagua.nl <http://lagua.nl/>
>> 
>> 
>> 
>> -- 
>> W.S. Hager
>> Lagua Web Solutions
>> http://lagua.nl <http://lagua.nl/>
> 
> 
> 
> 
> -- 
> W.S. Hager
> Lagua Web Solutions
> http://lagua.nl <http://lagua.nl/>

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