Subject: A Response on Completeness

From: Jim Clifford <[email-masked]>
Date: Thu, 26 Aug 93 11:56:30 EDT

I would like to reply (briefly) to Shashi Gadia's three comments about
completeness, since they refer, inaccurately, to my work with Croker
and Tuzhilin, and were broadcast a couple of days ago to the entire
tsql mailing list.

1:
>The formulations of completeness 
>on the basis of keys and the notions "grouped 
>and ungrouped" are neither very sound nor add 
>all that much to the expressive power.  

Our formulation of completeness is not at all based in any way on the
notion of keys. Moreover, we and others believe not only that the
distinction between temporally grouped and temporally ungrouped models
is sound, but that it sheds some much-needed light on the very real
differences between the multitude of models that have been proposed in
the literature of temporal databases.  In fact, we believe that the
"weak"-ness in the notion of "weak equivalence" introduced by Gadia is
explicated by the differences we point out between the modeling power
of grouped and ungrouped models.

2:
>At the workshop I demonstrated to Group B, a 
>serious flaw in the concept of time invariant 
>key to the extent that they are NOT really all 
>that invariant.  For example, the NAME attribute 
>seems invariant in the emp(NAME SALARY DEPT) 
>relation, but not invariant in management(DEPT NAME) 
>relation, if DEPT is chosen as the key.  

"time invariant" is a constraint that one may, or may not, wish to
impose upon an attribute, in the same way that one may impose the
constraint that an integer-valued attribute must be greater than 1000,
say. Thus it does not seem to make sense to say something "seems to be
time-invariant". If the model you are using allows the specification,
and provides the enforcement, of this type of constraint on an
attribute, then the attribute either is or is not so constrained.
Moreover, it is entirely possible (indeed typical) for an attribute,
such as NAME, to be constrained to have a certain behavior in one
relation, such as "emp", but not to be so constrained in another
relation, such as "management." I do not see what the "serious flaw"
is. Finally, it is worth pointing out that the whole notion of "time
invariant" makes little sense in ungrouped models, for the question
naturally arises, as Christian Jensen has pointed out, "invariant with
respect to what?" In grouped models, the answer is: "invariant within
the tuple." There is no such answer in ungrouped models, or at least
it is not as apparent.

3:
>I also exhibited how, by simply adding a system 
>alias table, the whole issue is cleanly resolved: modeling 
>(and hence querying) power is (strictly) increased; 
>and all this without making any changes to the syntax 
>and user-friendliness of our languages.

This concept was indeed mentioned, but only briefly and without
details, at the workshop. I would welcome seeing a complete treatment
of it to demonstrate that it will work.

Regards, 

--jim--