Re: Avoiding cycles in a directed graph

On 16/03/10 21:09, Tom Lane wrote:
> Tony Cebzanov<tonyceb@andrew.cmu.edu>  writes:
>> I'm okay with running the big, fat WITH RECURSIVE query in my insert
>> trigger if I have to -- it won't be great for performance, but I don't
>> expect this to be a frequent operation, so I'll accept the performance
>> hit if it works.
>
>> Unfortunately I can't even get that working.  Here's the (not at all
>> functional) trigger I've got right now, which only detects the cycle
>> *after* it's been inserted, which is of no help at all.  Any way I can
>> modify this to do the right thing?
>
> Run it in an AFTER trigger?
>
> If you don't expect this to be common, maybe you could fix the
> concurrency issue by taking a table-wide lock that locks out
> other writers.

Surely SELECT FOR UPDATE on the parents would be sufficient? If there's 
no overlap between (currently non-cyclic) graphs being altered then 
there can't be any conflict.

--   Richard Huxton  Archonet Ltd