Re: crash-safe visibility map, take three

Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
> Can we get away with not setting the LSN on the heap page, even though 
> we set the PD_ALL_VISIBLE flag? If we don't set the LSN, the heap page 
> can be flushed to disk before the WAL record, but I think that's fine 
> because it's OK to have the flag set in the heap page even if the VM bit 
> is not set.

Why is that fine?  It's certainly not fine from the standpoint of
someone wondering why his index-only scan performs so badly.

I think all this hair-splitting about cases where it's okay to have one
bit set and not the other is misguided.  To me, crash-safety of the VM
means that its copy of the page-header bit is right.  Period.  Yes, it
will cost something to ensure that; so what?  If we don't get more than
enough compensating performance gain from index-only scans, the whole
patch is going to end up reverted.
        regards, tom lane