On view updating

From: Alfredo Novoa <alfredo_at_ncs.es>
Date: 18 Sep 2004 06:24:31 -0700
Message-ID: <e4330f45.0409180524.3bc9d3fc_at_posting.google.com>

What do you think about this?

http://www.dbdebunk.com/page/page/1396086.htm

I can't agree more with this:

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I therefore have come to believe that "view updateability" should be allowed to happen IF AND ONLY IF the system has PRECISELY ONE WAY to "resolve" the user's request.
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In my opinion any approach to the view updating problem that is not based in logical inference is an absurd.

And a DBMS designed to make "inferences" like this:

A or B => C
C

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A and B

is something really awful and this is the behavior we can obtain from the application of Date & McGoveran's union update rules.

A DBMS that does this is a DBMS that lies, and this is not a very nice property for a DBMS IMO.

On the other hand, the rules are not able to infere updates so trivial like this:

var r relation { a integer, b integer } key { a, b }; var v view r where b = 1;

insert into v relation { tuple { a 1 } };

Any little child would infere that the base relvar update should be:

insert into r relation { tuple { a 1, b 1 } };

In the case of the join rules the problems are very numerous.

I agree on that not everything must be based in logic, but view updating definitely must be.

I sympathize a lot more with what McGoveran said like:

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3. A DBMS that supports the relational model cannot be "magic." It cannot compensate for ambiguity, intuit withheld information or assumptions, or correct expressions that violate the designer's or user's intent. If the designer fails to capture information in database design, if information is hidden from the user, or if the user incompetently fails to express their intent [Ed. Note: Or if the data language is not sufficiently expressive], it will certainly produce seemingly anomalous or "surprising" results when faced with these problems.
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A DBMS must not make wrong inferences, and whether a correct inference leads to surprising or counter intuitive results, it is a database designer's fault and not a DBMS flaw.

Regards
  Alfredo Received on Sat Sep 18 2004 - 15:24:31 CEST