Who conceptualized physical data independence before E.F. #Codd, Chris
#Date and Michael #Stonebreaker?

http://bit.ly/gibPP9

On 24/01/2011 11:57 AM, knorth wrote:

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.

On Jan 25, 11:04 am, paul c <anonym... (AT) not-for-mail (DOT) invalid> wrote:
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.

David BL wrote:

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.

On Jan 26, 12:38 am, Bob Badour <b... (AT) badour (DOT) net> wrote:
Fair enough.

Expanding on that, Codd proved the equivalence between his RA and a
particular predicate calculus which he defined in that paper. He
named it the Relational Calculus. It is strictly less powerful than
FOL.

If I'm not mistaken it is the most powerful predicate calculus that is
constrained by the requirement that under the intended interpretation
tuple variables have a clearly defined, finite range. This is
achieved by assuming the extensions of the monadic predicate constants
are finite (because it is assumed they are the base relations in the
database which must be finite), and there are restrictions on the use
of negation and disjunction in WFFs.