Re: Who conceptualized physical data independence before E.F. Codd, Chris Date and Michael Stonebreaker?
Date: Tue, 25 Jan 2011 08:38:17 -0800
Message-ID: <tM6dnWNnA-ziYaPQnZ2dnUVZ5v-dnZ2d_at_giganews.com>
David BL wrote:
> On Jan 25, 11:04 am, paul c <anonym..._at_not-for-mail.invalid> wrote:
>
>>On 24/01/2011 11:57 AM, knorth wrote:
>>
>>
>>>Who conceptualized physical data independence before E.F. #Codd, Chris
>>>#Date and Michael #Stonebreaker?
>>
>>>http://bit.ly/gibPP9
>>
>>What is this question about? No doubt Codd knew about Childs' work but
>>what direct comparison can apply? Codd showed a possible connection
>>between classical set ops and predicate logic. He didn't prescribe any
>>particular data structure (nor any particular domains for that matter).
>> The word 'physical' doesn't appear in his 1970 paper as far as I know.
>
>
> In Codd's '70 paper he uses the term "data independence", and I
> interpret it as physical data independence, where he talks about
> "independence of application programs ... from ... changes in data
> representation", or "... without superimposing any additional
> structure for machine representation purposes".
>
> What do you mean when you say Codd showed a possible connection
> between classical set ops and predicate logic? That only sounds like
> the idea of the predicate which is the indicator function of the set,
> or the set which is the extension of the predicate. The idea of that
> duality between sets and predicates has been around long before Codd.
> E.g. the axiom of (restricted) comprehension in set theory is
> essentially the idea that from a predicate one has an associated
> extension which is a set.
>
> In any case Child made it clear in his paper that a database could be
> regarded as recording sets and relations in particular (to be "pointer
> free") and set theoretic operators could form the basis for query on a
> database, and doing so provides a nice way of achieving data
> independence. I note as well that he defines operators on relations,
> including a "relative product" which is similar to a join. Also the
> duality between predicates and sets was implicit in his examples, such
> as
>
> M = { <x,y>: y is the mother of x }
>
> I would say that Codd's insight was his idea to restrict the entire
> database to just a set of named n-ary relations with *simple* domains
> and his small number of elegant and sufficient operators on relations.
Codd proved the equivalence between set algebra and predicate calculus in his 1972 paper. I am sure you can appreciate that apparent implicit duality is not a proof of duality. Received on Tue Jan 25 2011 - 17:38:17 CET