Re: Nested xacts: looking for testers and review

Alvaro Herrera wrote:
> On Fri, May 28, 2004 at 11:11:27PM -0400, Bruce Momjian wrote:
> > Alvaro Herrera wrote:
>
> > > I think this applies to all GUC variables, but I wonder if we want to
> > > save the value of each one at subtransaction start and recover it at
> > > abort? Things could easily get huge. Maybe only saving the ones that
> > > are different from the default value, and from the last saved value.
> >
> > We have an on-commit field in the guc structures to handle
> > commit/rollback settings. Do we need to extend that to subtransactions?
>
> Yes IMHO. I'm not sure actually _how_ should this be handled. Maybe
> the on-commit field should go away and be replaced by something more
> general (probably a stack, like everything else). Let me look at the
> code.
>
> > I don't think you can save off only the defaults in an efficient manner.
>
> What do you mean by efficient? Space efficient? It will be much more
> efficient to save only the changed values. Time efficient? We will
> have to traverse the whole list anyway, whether we only save the changed
> values or all of them.
>
> Remember, we already traverse the whole list of shared buffers, the
> whole CatCache, the whole Relcache, maybe do some repallocs, and lots of
> other stuff. Traversing the whole GUC array does not seem all that
> expensive to me ... After all, we are saving lots of I/O if subxacts
> are used correctly (try, rollback, try again -- you save the XLog for
> the first try.)

My comment was based on the fact that guc already does some special
_saves_ when you change a value and triggers some stuff on xact end. I
was just thinking it would be cleaner to use that infrastructure rather
than do a scan not knowing if any GUC will change or not, but if a scan
is easier, I think that would be fine.

--
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