Re: Recursive containment of composite types

On Mar 28, 2011, at 10:43 AM, Tom Lane wrote:
> Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
>> On 03/28/2011 11:14 AM, Tom Lane wrote:
>>>> I think the most straightforward and reliable fix for this would be to
>>>> forbid recursive containment of a rowtype in itself --- ie, the first
>>>> ALTER should have been rejected. Can anyone think of a situation where
>>>> it would be sane to allow such a thing?
>
>> I think we should forbid it for now. If someone comes up with a) a good
>> way to make it works and b) a good use case, we can look at it then. I
>> expect the PostgreSQL type system to be a good deal more constrained
>> than a general in-memory programming language type system. If lack of
>> working type recursion were a serious problem surely we'd have seen more
>> squawks about this by now.
>
> The immediate issue in CheckAttributeType() could be fixed by tracking
> which types it was processing and not recursing into an already-open
> type. Which, not at all coincidentally, is 90% the same code it'll need
> to have to throw error. The issue for really "making it work" is how do
> we know if there are any other places that need a recursion defense?
> I'm pretty sure that find_composite_type_dependencies would, and I don't
> know where else there might be a hidden assumption that column
> references don't loop. So I think that it's mostly about testing rather
> than anything else. If I were fairly confident that I knew where all
> the risk spots were, I'd just fix them rather than trying to forbid the
> construction.

Perhaps forbid it for now (for safety) but provide the reporter with a patch that would unblock them so they can do further testing?

The concept is certainly interesting so it'd be nice to support it if we could. It seems like a good fit for things like storing tree structures. Though, if the type is still hauling around a heap tuple header I think they'll find the performance of this whole thing to be rather lacking... :(
--
Jim C. Nasby, Database Architect jim(at)nasby(dot)net
512.569.9461 (cell) http://jim.nasby.net