Re: DDL Damage Assessment

On Thu, Oct 2, 2014 at 1:52 PM, Claudio Freire <klaussfreire@gmail.com> wrote:
> The explain would show the AccessExclusiveLock, so it would be enough
> for a heads-up to kill all idle-in-transaction holding locks on the
> target relation (if killable, or just wait).

I think that there are very few problems with recognizing when an
AccessExclusiveLock is needed or not needed. The exceptions to the
rule that DDL needs such a lock are narrow enough that I have a hard
time believing that most people think about it, or even need to think
about it. I wish that wasn't the case, but it is.

> Granted, it's something that's not easily automatable, whereas a nowait is.
>
> However, rather than nowait, I'd prefer "cancellable" semantics, that
> would cancel voluntarily if any other transaction requests a
> conflicting lock, like autovacuum does.

I think the problem you'll have with NOWAIT is: you have an error from
having to wait...what now? Do you restart? I imagine this would
frequently result in what is effectively lock starvation. Any old
AccessShareLock-er is going to make our migration tool restart. We'll
never finish.

-- 
Peter Geoghegan