Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?

From: Jan Hidders <>
Date: 16 Feb 2003 02:53:31 -0800
Message-ID: <>

"Paul Vernon" <paul.vernon_at_ukk.ibmm.comm> wrote in message news:<b2m4he$eli$>...
> "Lauri Pietarinen" <> wrote in message
> [snip]
> > I am just trying to get a hold on this matter, what is it exactly that
> > Date get's wrong or missunderstands.
> > Are they trying to achieve different goals? Maybe my head is a bit
> > thick but I am not convinced.
> For what it is worth, from this discussion I am still convinced that bags are
> a bad model for users, because to quote from Date's second article,
> "[with bags] the range of possible results that can be obtained from any given
> database is
> vastly expanded, though not usefully so"
> "[with bags] the range of logically distinct query expressions that can be
> formulated is
> vastly expanded, too, though again not usefully so"
> In otherwords, Occams razor comes into play.

Not to be pedantic but Ockham's razor is for scientific theories and the relational model is no such thing. However, it is an engineering method and as
such KISS applies, which is related,

Anyway, the above quotes by Date are strictly speaking correct but in this context misleading. The extra queries you can do are isomorphic to queries that could already be done without bags. So in terms of query optimization no extra problems are introduced that weren't there already.

> As an internal bag algebra might well help implementations provide better
> performance, I can see arguments for exposing such an algebra to users to
> possibly make the implementators life easier.

I would not expose the algebra anyway, it is there for implementation purposes. The users should be exposed to a bag *calculus*. But this is another matter.

> However, that is the tail wagging the dog.
> I can only see costs in a bag algebra for users.

Since when is better query optimization a cost? And that bags are harder to understand than sets is hardly what I would call a scientific theory. My personal experience as a professional teacher suggests otherwise, and the arguments I have seen to support it are hardly any better than handwaving.

  • Jan Hidders, who is signing off now because his plane will be leaving in a few hours.
Received on Sun Feb 16 2003 - 11:53:31 CET

Original text of this message