Re: more closed-world chatter

From: Jon Heggland <jon.heggland_at_idi.ntnu.no>
Date: Fri, 11 May 2007 13:03:33 +0200
Message-ID: <f21ilb$gse$1@orkan.itea.ntnu.no>

Marshall wrote:
> If we are joining X with DUM, then there are necessarily no
> attributes in common, so we would not expect to perturb
> the attribute types from X.

So you won't consider each type of the result to be additionally constrained by "and false", thus leading to the bottom type?

>> Is generalized minus the same as semiminus?

>
> As Vadim mentioned, it is more traditionally called
> antijoin. (And has he has also mentioned, it's an
> unfortunate name since it's the inverse not of
> join but of generalized union.)

That isn't obvious to me, but then I have always had trouble understanding those inverses you and Vadim are talking about.

-- 
Jon

Received on Fri May 11 2007 - 06:03:33 CDT