Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Mon, 4 Jul 2005 20:33:24 -0400
Message-ID: <DaWdnZZvzMvNSFTfRVn-uw_at_comcast.com>
"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
news:64hye.137293$VX3.7329860_at_phobos.telenet-ops.be...
> VC wrote:
>> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
>> news:sxRxe.136269$3i5.7167004_at_phobos.telenet-ops.be...
>>
>>>VC wrote:
>>>
>>>>"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
>>>>news:xxBxe.135773$vq.7300203_at_phobos.telenet-ops.be...
>>>>
>>>>
>>>>>VC wrote:
>>>>>
>>>>>
>>>>>>"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
>>>>>>news:eHrxe.135259$JD6.7251058_at_phobos.telenet-ops.be...
>>>>>>
>>>>>>
>>>>>>
>>>>>>>vc wrote:
>>>>>>>
>>>>>
>>>>>Anyway, why do you want specifically an algebra? You don't need one for
>>>>>asking queries, not for defining views and you don't need one for query
>>>>>optimization, so why bother?
>>>>
>>>>Please clarify whether tour are talking about the RM views and queries
>>>>or about something else.
>>>
>>>All of them, really. FDM-like models, OODB data models, and also the RM.
>>
>> OK, we want specifically an algebra (RA) primarily for two practical
>> reasons.
>>
>> (A) It's trivial to compose relational algebra operators (create nested
>> queries) thanks to the closure property. One can create arbitrarily
>> complex queries by using several simple and easily understood relational
>> operators (join, projection, etc). I imagine similar freedom is not
>> available to the user even with modern network/graph query languages
>> (whatever those might be).
>
> Well, it is available, most of those languages are orthogonal.
For example ? One non-relational language example would suffice.
>
>> (B) The same freedom to re-arrange RA operators is extremely useful for
>> query optimization (contrary to your opinion). Internally, the query
>> processor decomposes the original query into a set of RA operators and
>> then tries to rewrite the query so that its execution were most effective
>> in terms of resorces (CPU, IO, etc). In other words, the query
>> processor creates an efficient execution plan. I believe this kind of
>> flexibility is not possible with alternative data models ue to lack of
>> closure.
> > Your belief is wrong. Every rearrangement can also be expressed in a > calculus.
In *what* calculus ? In relational, sure, since it's equivalent to the r.a. What other calculus(i) did you have in mind ? Please provide an example of such rearrangement.
>>>>Well, apparently, I am not as smart as most of your students are.
>>>>Please provide a definition of "conceptual object type".
>>>
>>>A unary predicate.
>>
>> If so, what would it mean to say that object O is of conceptual type T
>> given that T is a unary predicate ?
>
> T(O) holds.
What does it mean ? What's the "T(O)" domain of interpratation ?
>>>>We can, with the additional restriction that the elements of the domains
>>>>>>OK. Firstly, why do we care about 'value representation' at all ?
>>>>>
>>>>>Because otherwise we couldn't indicate in an ORM schema that the name
>>>>>of a department is represented as a string.
>>>>
>>>>Cannot we just use the notion of data type (aka domain) instead ?
>>>
>>>must have some kind of lexical representation.
>>
>> Please define "lexical representation".
>
> A finite string over some finite alphabet.
Why do we care about a finite string over some finite alphabet representing a value ?
Thanks.
>
> -- Jan Hidders
Received on Tue Jul 05 2005 - 02:33:24 CEST