Let's say you have a parent relations as (parent, child). Now you want
to compute all ancestors from this relation that is technically spoken
you need a transitive closure.
In logic this looks like:
ancestor(X,Y) :- parent(X,Y).
ancestor(X,Y) :- ancestor(X,Z), parent(Z,Y).
X is ancestor of Y if X is parent of Y or X is ancestor of Z who is
parent of Y.
In POSTQUEL this is written like this (note I'm not sure about correct
POSTQUEL syntax):
retrieve into ancestor(p.x,p.y) from p in parent
retrieve* into ancestor(a.x,p.y) from a in ancestor, p in parent where
a.y=p.x.
You see the second line generates new tuples every time it's executed
until there is no more data deductable.
Michael
--
Dr. Michael Meskes, Projekt-Manager | topystem Systemhaus GmbH
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> ----------
> From: Thomas G. Lockhart[SMTP:lockhart@alumni.caltech.edu]
> Sent: Freitag, 20. Februar 1998 17:44
> To: Meskes, Michael
> Cc: 'Vadim B. Mikheev'; PostgreSQL Hacker
> Subject: Re: [HACKERS] Recursive queries?
>
> > But such a statement is executed only once, isn't it?
> >
> > Postquel did execute a retrieve* into as long as it was able to
> generate
> > new data.
>
> OK, I'll bite :)
> Can you give an example of this? How does new data become available in
> the
> middle of a transaction? Just curious...
>
