Re: a union is always a join!
Date: Wed, 4 Mar 2009 05:30:51 -0500
"Brian Selzer" <brian_at_selzer-software.com> wrote in message
> "paul c" <toledobythesea_at_oohay.ac> wrote in message
>> rpost wrote:
>>> As I already said in my previous message, I see the elegance,
>>> but at the same time I find elegance in Codd's use of two kinds
>>> of operations. Obviously if you rewrite to Codd's algebra (or do
>>> something equivalent to that) you're not going to lose efficiency
>>> on the cases where this is possible. But just rejecting the rest
>>> is only going to confuse the user. Why not be honest and admit
>>> that we really expect, in our implementations, relational queries
>>> to work on finite, typically small relations, that WORKS_IN is one
>>> and PLUS is not, and that NOT doesn't do so?
>> In the interest of keeping the pot stirred, here are some other
>> provocations (basically one of my same old tired recurring themes) .
>> As Fabian P and others have pointed out, a table (or should that be
>> R-table?) that involves nulls obviously involves more than one of Codd's
>> original relations. Given a smart enough implementation a relation that
>> is defined using <OR> could be replaced by two or more relations (I seem
>> to remember McGoveran stating similar somewhere or other. I'm guessing
>> that Codd wouldn't have agreed but my hunch is that his objection would
>> have been on practical, not theoretical grounds, so perhaps he would not
>> have objected if somebody had made a practical implementation). As long
>> as the closed-world-assumption is followed, a relation that is produced
>> using the <NOT> operator doesn't generally need to be materialized
>> because queries against it can be transformed as queries against its own
>> negation. I'd say the importance of <NOT> has nothing to do with
>> finiteness, rather that it parallels boolean algebra's NOT and thus
>> allows a algebra to define retraction, aka 'deletion'.
>> <OR> is more interesting (to me) than <NOT>. Without <OR>, RT has no way
>> to 'add' a tuple to a relation variable. When one uses a language's
>> "INSERT" verb, it must effect the logical equivalent of <OR>.
> I think you're on the right track if you can just separate what <OR> is
> from what is an application of <OR>. Of course, this particular
> application of <OR> is not limited to just INSERT, but also UPDATE and
> Relational Calculus, and therefore equivalent mechanisms such as Codd's or
> D&D's algebras, is closely related to if not based upon first order
> predicate logic. But while first order logic is fine for querying the
> picture of the world that is represented by a given database value, it is
> not sufficient for either the series of pictures represented by a
> succession of database values, or the series of events that describe not
> just what is changing between pairs of successive pictures, but exactly
> how what is changing is different.
> This is the way I see it: That which states what is the case is the
> disjunction of all and only true propositions. Divide that into two
> parts: one that states what has been the case (not necessarily what had
> been the case--just that which obtained since the last change), and one
> that states what is happening. Whenever nothing is happening, what has
> been the case /is/ the case. For example, if Joe has been second in line
> at the bank and nothing is happening, then Joe is still second in line.
> On the other hand, if the person who has been first in line is now being
> served, then Joe is no longer second in line but is instead first in line.
> The disjunction of the two statements: "Joe has been second in line" and
> "Joe is no longer second in line but is instead first in line" yields "Joe
> is first in line." In symbols,
> from p \/ (~p \/ q), derive (p \/ ~p) \/ q
> since (p \/ ~p) is vacuously true, eliminate it and what is left is just q
Technically, (p \/ ~p) is not /vacuously/ true, it just conveys no information. Still, such expressions must not appear as disjuncts in the statement of what is the case.
> Now let's apply that to relational theory: the database states what has
> been the case; a transition states what is happening. The logical sum of
> these two statements asserts what is the case, and is therefore what is
> becoming the database: though there may be many databases that could be
> the database, only one of them can actually be the database at a time.
> So the application of <OR> to combine the statement of what has been the
> case with the statement of what is happening is a fair description of a
> database update, but it is not limited to just "INSERT." The disjunction,
> p \/ ~p, when it appears in the logical sum of what has been the case and
> what is happening can be thought of as the equivalent of a "DELETE," and
> the disjunction, (p /\ q) \/ (p /\ ~q) \/ (p /\ r), the equivalent of an
> "UPDATE;" only the disjunction, False \/ p, would be the equivalent of an
>> Codd's unadorned relation is a conjunction of simple propositions (by
>> simple I mean each proposition contains no logical connectors). If a
>> relation variable has the 'quality' of being a 'view' (or 'green' as I
>> like to joke), some people say to the effect that the view is a
>> conjunction of disjunctions (propositions involving the logical connector
>> 'OR'). Whereas if the relation variable is 'base', they say it is just a
>> conjunction of simple propositions. For years, I've been trying to
>> understand how they can say this at the same time as they say that users
>> should preferably see no difference between base and virtual/view
>> relation variables (which I agree with, just as I would agree that there
>> should be no appreciable difference between 'red' and 'green' relvars.
>> Their argument then proceeds to claim that 'inserts' to some union views
>> are indeterminate while 'inserts' to an equal 'base' view aren't. I'd say
>> that if they are right, then we shouldn't be able to 'insert' to any
>> variable unless it points to an empty relation which would mean that no
>> relation could have more than one tuple - that bizarre situation would
>> have exactly one advantage - no questions about finiteness!
Received on Wed Mar 04 2009 - 11:30:51 CET