Re: heap vacuum & cleanup locks

Robert Haas <robertmhaas@gmail.com> writes:
> Interesting idea.  I think in general we insist that you must have a
> buffer content lock to inspect the tuple visibility info, in which
> case that would be safe.  But I'm not sure we do that absolutely
> everywhere.  For instance, just last night I noticed this:

>                         /*
>                          * If xmin isn't what we're expecting, the
> slot must have been
>                          * recycled and reused for an unrelated tuple.
>  This implies that
>                          * the latest version of the row was deleted,
> so we need do
>                          * nothing.  (Should be safe to examine xmin
> without getting
>                          * buffer's content lock, since xmin never
> changes in an existing
>                          * tuple.)
>                          */
>                         if

Hmm ... I think that code is OK but the comment needs work.  Here we are
necessarily looking for a pretty recent value of xmin (it has to be
later than GlobalXmin), so there's no need to worry that it might get
changed to FrozenXID.
        regards, tom lane