On Mon, 15 Jun 2015 11:03:17 +1000
<david at andl.org> wrote:
> >>>Unless the recursion is circular, I don't see how an SQL query
> >>>over a finite database could fail to terminate.
>
> What does this mean? It is trivial to write a recursive CTE that does
> not terminate, and the property of "circularity" is not what makes the
> difference.
Hmm, for a correctly written recursive query not to terminate, is it not
a requirement that the data contain a cycle? I can't prove it, but no
counterexample springs to mind.
In the positive: a correct recursive query always terminates if the
data represent a directed acyclic graph.
By "correct" I mean the CTE expresses a recursive relation. If you
recurse over
with R (a, b) as (select 1 as a, 1 as b)
you have no right to expect termination. But you might be able to fry
an egg on the processor.
--jkl
