Re: Parallel copy

Hi,

On 2020-06-04 08:10:07 +0530, Amit Kapila wrote:
> On Thu, Jun 4, 2020 at 12:09 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > > I strongly disagree with the idea of "just sync(ing) it up at the end
> > > of parallelism". That seems like a completely unprincipled approach to
> > > the problem. Either the command counter increment is important or it's
> > > not. If it's not important, maybe we can arrange to skip it in the
> > > first place. If it is important, then it's probably not OK for each
> > > backend to be doing it separately.
> >
> > That scares me too. These command counter increments definitely aren't
> > unnecessary in the general case.
> >
>
> Yeah, this is what we want to understand? Can you explain how they
> are useful here? AFAIU, heap_lock_tuple doesn't use commandid while
> storing the transaction information of xact while locking the tuple.

But the HeapTupleSatisfiesUpdate() call does use it?

And even if that weren't an issue, I don't see how it's defensible to
just randomly break the the commandid coherency for parallel copy.

Greetings,

Andres Freund