Re: Idempotence and "Replication Insensitivity" are equivalent ?
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Mon, 25 Sep 2006 22:49:25 GMT
Message-ID: <V1ZRg.39749$9u.336822_at_ursa-nb00s0.nbnet.nb.ca>
>> paul c wrote:
>>
>>> Bob Badour wrote:
>>>
>>>> paul c wrote:
>>>>
>>>>> David Cressey wrote:
>>>>>
>>> ...
>>>
>>>>> But I'm also thinking that when you say 'project a relation onto
>>>>> its attributes', if such a thing were permitted by some RM impl'n,
>>>>> what *could* actually happen is that a relation with a single
>>>>> relation-valued attribute would be formed and I suppose that
>>>>> attribute's 'type' would be the name of the relation. But join is
>>>>> usually the operator we expect to be able to undo a projection, so
>>>>> if an impl'n did this, then I suppose it might want to undo the
>>>>> rva-creating projection, and that might entail that it also have a
>>>>> way of equating a relation with several attributes against a
>>>>> single-attribute rva equivalent.
>>>>
>>>> Such as the relational equality operation?
>>>
>>> Yes, but I suspect when I come up with examples, some
>>> trouble/ambibuity will show up.
>>>
>>>>> In this admittedly oddball view of things, I wonder if the name of
>>>>> an rva really matters? That's as far as I've got.
>>>>
>>>> What's oddball about it?
>>>
>>> I wasn't clear - I didn't mean the rva can have any (unique) name we
>>> choose, just wondering if there are times when an rva could be
>>> unnamed without risk - that's what I meant by oddball.
>>
>> If an attribute has no name, how are we to refer to it?
Date: Mon, 25 Sep 2006 22:49:25 GMT
Message-ID: <V1ZRg.39749$9u.336822_at_ursa-nb00s0.nbnet.nb.ca>
paul c wrote:
> Bob Badour wrote: >
>> paul c wrote:
>>
>>> Bob Badour wrote:
>>>
>>>> paul c wrote:
>>>>
>>>>> David Cressey wrote:
>>>>>
>>> ...
>>>
>>>>> But I'm also thinking that when you say 'project a relation onto
>>>>> its attributes', if such a thing were permitted by some RM impl'n,
>>>>> what *could* actually happen is that a relation with a single
>>>>> relation-valued attribute would be formed and I suppose that
>>>>> attribute's 'type' would be the name of the relation. But join is
>>>>> usually the operator we expect to be able to undo a projection, so
>>>>> if an impl'n did this, then I suppose it might want to undo the
>>>>> rva-creating projection, and that might entail that it also have a
>>>>> way of equating a relation with several attributes against a
>>>>> single-attribute rva equivalent.
>>>>
>>>> Such as the relational equality operation?
>>>
>>> Yes, but I suspect when I come up with examples, some
>>> trouble/ambibuity will show up.
>>>
>>>>> In this admittedly oddball view of things, I wonder if the name of
>>>>> an rva really matters? That's as far as I've got.
>>>>
>>>> What's oddball about it?
>>>
>>> I wasn't clear - I didn't mean the rva can have any (unique) name we
>>> choose, just wondering if there are times when an rva could be
>>> unnamed without risk - that's what I meant by oddball.
>>
>> If an attribute has no name, how are we to refer to it?
> > By its type (at least when the type is unique within a relation/relvar), > which for an rva would be the name of the original relation/relvar.
In other words, use the name of the type as a name for the attribute. Doesn't that mean the attribute then has a name? What if another attribute has the same name as the type? What if two unnamed attributes have the same type? Received on Tue Sep 26 2006 - 00:49:25 CEST