Re: heterogeneous DBs
brayman@zuben.ca.boeing.com (Bill Brayman)
Date: Mon, 13 Sep 93 17:39:40 PDT
From: brayman@zuben.ca.boeing.com (Bill Brayman)
Message-id: <9309140039.AA14216@zuben.ca.boeing.com>
To: interlingua@ISI.EDU
Subject: Re: heterogeneous DBs
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I'm not speaking for Jeffrey Van Baalen, but I suspect that there is a
misunderstanding over 'semantics' here. Brayman uses 'semantics' to
refer to what in AI is often called
knowledge representation, ie some
implemented system which somehow encodes knowledge in a formalism:
semantics-1.
Others - perhaps Van Ballen - use 'semantics' to refer to
something like model theory, ie an account of how these formalisms
could relate to a possible world: semantics-2.
The former is a
computational achievement, the latter is a theoreticians' tool. In the
latter perspective, declarative languages such as KIF and CG and
database languages don't constitute semantics: they are things that
have, or can be given, a semantics. And semantics, in this sense, are
not things that one would usually expect to find implemented in
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executable form. (Although interesting things might be achieved by so
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doing, that would not be doing knowledge representation. For example,
inference can be usefully viewed as a process of showing that models
don't exist by trying to construct them sufficiently systematically.)
A reason that many AI people are 'shocked' by a lack of semantics-2 is
that unless such an account is provided, there is no exact way to say
what the expressions of a representational formalism might mean, so
talking about translation between one and onother, for example, becomes
difficult to make precise enough for their tastes. This applies just as
well to the formalisms used in the DB itself as to those proposed to
encode their semantics-1; in fact to anything in a computer which is
supposed to somehow encode knowledge.
My aim is not to start another quarrel, but to try to avoid confusion.
Can we find a terminology which we can use to all communicate with one
another without misunderstanding? Let me tentatively suggest that we
might speak of 'semantic representations' for semantics-1 and 'semantic
interpretations' for semantics-2. This allows me to slip naturally into
'representation' and 'interpretations'. Is it similarly acceptable to
someone with a database background?
Pat Hayes
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Pat Hayes makes an interesting distinction between semantic
representations and semantic interpretations. I would like to make a
couple of comments while I take some time to ponder Pat's dichotomy.
First, it strikes me as strange that representations are more likely to
be executable or more usefully so, than interpretations.
Secondly, if we consider translations under this dichotomy, then
database people are interested in both aspects.
A common activity these days is to translate from a structured file
system or database system into a relational database system. Each type
of database system requires the user to commit to some representational
scheme. The job of the translator is to determine how to map from one
to the other while maintaining the meaning of the original. That seems
to me to require dealing with the interpretability of each database.
With the older databases, not much of that form of semantics is in the
database, but is, rather, in the software that operates on the data.
With relational databases, interpretability is partially in the
database. Take for example functional dependency. A relational
database tuple carries the implicit semantics that, loosely speaking,
given a key attribute value, the other attribute values within a tuple
are functionally dependent on that key value. The semantics of this is
that the database record represents the world in that, where one
attribute value determines another in the database record, the
corresponding key characteristic of a real world object determines
another dependent characteristic of that object.
to repeat Pat:
something like model theory, ie an account of how these formalisms
could relate to a possible world: semantics-2.
Based on this, I would say that functional dependency is an
interpretation semantics in Pat Hayes sense.
regards,
Bill Brayman