Re: Can relational alegbra perform bulk operations?
Date: Tue, 29 Sep 2009 12:25:42 -0700 (PDT)
On Sep 29, 11:23 am, Banana <Ban..._at_Republic.com> wrote:
> I'm new to relational theory, having read a C.J. Date's book but I worry
> that I may have picked up mistaken impression about the relational
> theory and thus want to validate whether my understanding is accurate or
> Prior to reading the book, I've always had understood that anything we
> ... We can see how much relational
> algebra/calculus can help us optimize any kind of queries by
> transforming the expression.
This is not really the case. All these transformations are ad-hock. If you try proving, say, "pushing selection through projection" you would get bogged in set notation with awkward attribute indexing, and even if succeed actually proving it, it would hardly result in any insight.
> But... I don't see any means within the relational algebra that provides
> a way of evaluating multiple tuples in one go. Considering that a
> relation is essentially a collection of propositions conforming to a
> given predicate, it seems necessary to evaluate each tuples to determine
> whether they should participate in the join or not, satisfy the restrict
> condition and other things. For a lack of better terms, there is no
> "sieve" we can employ to evaluate a set of tuples in one go.
> Is the preceding paragraph accurate?
It is too vague. Algebras, in general, don't care about the structure of their elements; relational algebra, in particular, doesn't refer to tuples. (BTW the fact that it refers to attributes is a flaw -- c.d.t regulars might have already guessed I'm about to sing the familiar relational lattice tune:-) Received on Tue Sep 29 2009 - 21:25:42 CEST