Re: Removing PD_ALL_VISIBLE

On Thu, 2013-01-17 at 15:25 +0530, Pavan Deolasee wrote:
> Now that I look at the patch, I wonder if there is another fundamental
> issue with the patch. Since the patch removes WAL logging for the VM
> set operation, this can happen:
>
Thank you. I think I was confused by this comment here:

"When we *set* a visibility map during VACUUM, we must write WAL. This
may seem counterintuitive, since the bit is basically a hint: if it is
clear, it may still be the case that every tuple on the page is visible
to all transactions; we just don't know that for certain. The
difficulty is that there are two bits which are typically set together:
the PD_ALL_VISIBLE bit on the page itself, and the visibility map bit.
If a crash occurs after the visibility map page makes it to disk and
before the updated heap page makes it to disk, redo must set the bit on
the heap page. Otherwise, the next insert, update, or delete on the heap
page will fail to realize that the visibility map bit must be cleared,
possibly causing index-only scans to return wrong answers."

Which lead me to believe that I could just rip out the WAL-related code
if PD_ALL_VISIBLE goes away, which is incorrect. But the incorrectness
doesn't have to do with the WAL directly, it's because the VM page's LSN
is not bumped past the LSN of the related heap page cleanup, so it can
be written too early.

Of course, the way to bump the LSN is to write WAL for the
visibilitymap_set operation. But that would be a very simple WAL
routine, rather than the complex one that exists without the patch.

I suppose we could even try to bump the LSN without writing WAL somehow,
but it doesn't seem worth reasoning through that (setting a VM bit is
rare enough).

Regards,
Jeff Davis