Re: Reviewing freeze map code

On 2016-06-23 18:59:57 -0400, Alvaro Herrera wrote:
> Andres Freund wrote:
>
> > I'm looking into three approaches right now:
> >
> > 3) Use WAL logging for the already_marked = true case.
>
>
> > 3) This approach so far seems the best. It's possible to reuse the
> > xl_heap_lock record (in an afaics backwards compatible manner), and in
> > most cases the overhead isn't that large. It's of course annoying to
> > emit more WAL, but it's not that big an overhead compared to extending a
> > file, or to toasting. It's also by far the simplest fix.
>
> I suppose it's fine if we crash midway from emitting this wal record and
> the actual heap_update one, since the xmax will appear to come from an
> aborted xid, right?

Yea, that should be fine.

> I agree that the overhead is probably negligible, considering that this
> only happens when toast is invoked. It's probably not as great when the
> new tuple goes to another page, though.

I think it has to happen in both cases unfortunately. We could try to
add some optimizations (e.g. only release lock & WAL log if the target
page, via fsm, is before the current one), but I don't really want to go
there in the back branches.

Andres