On 3/16/10 6:22 PM, Tom Lane wrote:
> Richard Huxton writes:
>
>
> Um, what if the cycle is being formed from whole cloth? For instance
> T1 inserts an edge A->B while T2 is inserting B->A. There are no
> pre-existing rows to lock, but there will still be a cycle after they
> both commit.
For
Richard Huxton writes:
> On 16/03/10 21:09, Tom Lane wrote:
>> 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 b
On 16/03/10 21:09, Tom Lane wrote:
Tony Cebzanov 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.
Unfortu
Tony Cebzanov 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 w
On 3/16/10 4:34 PM, Tom Lane wrote:
> The same kind of problem exists for unique and foreign key constraints,
> both of which use low-level locking mechanisms to catch such cases.
> There's no way that I can see to express the "no cycle" constraint as a
> uniqueness constraint unfortunately. You c
Richard Huxton writes:
> On 16/03/10 19:45, Tony Cebzanov wrote:
>> That's great to avoid looping forever on queries, but what about
>> preventing anyone from inserting edges that would create cycles in the
>> first place? I reckon I'll need a trigger of some sort, but nothing
>> I've tried has e
On 16/03/10 19:45, Tony Cebzanov wrote:
I'm using the following table to represent an acyclic directed graph:
[snip]
I see there is an example in the online docs for detecting cycles in
recursive queries, and I've adapted the example query to the table above:
[snip]
That's great to avoid loop
