Re: preserving forensic information when we freeze

On 2013-12-11 14:17:27 -0500, Robert Haas wrote:
> > But in which cases would that actually be slower? There'll be no
> > additional code executed if the hint bits for frozen are set, and in
> > case not it will usually safe us an external function call to
> > TransactionIdPrecedes().
>
> Dunno. It's at least more code generation.

IMO it looks cleaner and is less error prone, so an additional
instruction or two in the slow path doesn't seem a high price. And we're
really not talking about more.

But I won't do more than roll my eyes loudly if you decide to go ahead
with your version as long as you change rewriteheap.c accordingly.

> >>> > I think once we have this we should start opportunistically try to
> >> > freeze tuples during vacuum using OldestXmin instead of FreezeLimit if
> >> > the page is already dirty.
> >>
> >> Separate patch, but yeah, something like that. If we have to mark the
> >> page all-visible, we might as well freeze it while we're there. We
> >> should think about how it interacts with Heikki's freeze-without-write
> >> patch though.
> >
> > Definitely separate yes. And I agree, it's partially moot if Heikki's
> > patch gets in, but I am not sure it will make it into 9.4. There seems
> > to be quite some work left.
>
> I haven't heard anything further from Heikki, so I'm thinking we
> should proceed with this approach.

Yea, I think by now it's pretty unlikely to get into 9.4. I hope for
early 9.5 tho.

What's your plan to commit this? I'd prefer to wait till Alvaro's
freezing changes get in, so his patch will look the same in HEAD and
9.3. But I think he plans to commit soon.

> If we also
> handle the vacuum-dirtied-it-already case as you propose here, I think
> we'd have quite a respectable improvement in vacuum behavior for 9.4,
> even without Heikki's stuff.

Yes, and it seems like a realistic goal, so let's go for it.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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