On Tue, Jul 03, 2001 at 05:11:46PM -0400, Tom Lane wrote:
> ncm(at)zembu(dot)com (Nathan Myers) writes:
> > On Mon, Jul 02, 2001 at 09:40:25PM -0400, Tom Lane wrote:
> >> 4. It is considered OK to update tuple commit status bits (ie, OR the
> >> values HEAP_XMIN_COMMITTED, HEAP_XMIN_INVALID, HEAP_XMAX_COMMITTED, or
> >> HEAP_XMAX_INVALID into t_infomask) while holding only a shared lock and
> >> pin on a buffer. This is OK because another backend looking at the tuple
> >> at about the same time would OR the same bits into the field, so there
> >> is little or no risk of conflicting update; what's more, if there did
> >> manage to be a conflict it would merely mean that one bit-update would
> >> be lost and need to be done again later.
> > Without looking at the code, this seems mad. Are you sure?
> Yes. Those status bits aren't ground truth, only hints. They cache the
> results of looking up transaction status in pg_log; if they get dropped,
> the only consequence is the next visitor to the tuple has to do the
> lookup over again.
> Changing any other bits in t_infomask requires exclusive lock, however.
1 load t_infomask
2 or XMIN_INVALID
4 load t_infomask
5 or MOVED_IN
6 store t_infomask
8 store t_infomask
Here, backend B is a good citizen and locks while it makes its change.
Backend A only means to touch the "hint" bits, but it ends up clobbering
the MOVED_IN bit too.
Also, as hints, would it be Bad(tm) if an attempt to clear one failed?
That seems possible as well.
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