Re: Idempotence and "Replication Insensitivity" are equivalent ?
From: paul c <toledobythesea_at_oohay.ac>
Date: Mon, 25 Sep 2006 22:29:54 GMT
Message-ID: <CLYRg.31208$5R2.31073_at_pd7urf3no>
>> 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 22:29:54 GMT
Message-ID: <CLYRg.31208$5R2.31073_at_pd7urf3no>
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.
p Received on Tue Sep 26 2006 - 00:29:54 CEST