Re: Emit fewer vacuum records by reaping removable tuples during pruning

I had already written the patch for immediate reaping addressing the
below feedback before I saw the emails that said everyone is happy
with using hastup in lazy_scan_[no]prune() in a preliminary patch. Let
me know if you have a strong preference for reordering. Otherwise, I
will write the three subsequent patches on top of this one.

On Tue, Jan 9, 2024 at 2:00 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Tue, Jan 9, 2024 at 11:35 AM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
> > The easiest solution would be to change the name of the parameter to
> > heap_page_prune_execute()'s from  "no_indexes"  to something like
> > "validate_unused", since it is only used in assert builds for
> > validation.
>
> Right.
>
> > However, though I wish a name change was the right way to solve this
> > problem, my gut feeling is that it is not. It seems like we should
> > rely only on the WAL record itself in recovery. Right now the
> > parameter is used exclusively for validation, so it isn't so bad. But
> > what if someone uses this parameter in the future in heap_xlog_prune()
> > to decide how to modify the page?
>
> Exactly.
>
> > It seems like the right solution would be to add a flag into the prune
> > record indicating what to pass to heap_page_prune_execute(). In the
> > future, I'd like to add flags for updating the VM to each of the prune
> > and vacuum records (eliminating the separate VM update record). Thus,
> > a new flags member of the prune record could have future use. However,
> > this would add a uint8 to the record. I can go and look for some
> > padding if you think this is the right direction?
>
> I thought about this approach and it might be OK but I don't love it,
> because it means making the WAL record bigger on production systems
> for the sake of assertion that only fires for developers. Sure, it's
> possible that there might be another use in the future, but there
> might also not be another use in the future.
>
> How about changing if (no_indexes) to if (ndead == 0) and adding a
> comment like this: /* If there are any tuples being marked LP_DEAD,
> then the relation must have indexes, so every item being marked unused
> must be a heap-only tuple. But if there are no tuples being marked
> LP_DEAD, then it's possible that the relation has no indexes, in which
> case all we know is that the line pointer shouldn't already be
> LP_UNUSED. */

Ah, I like this a lot. Attached patch does this. I've added a modified
version of the comment you suggested. My only question is if we are
losing something without this sentence (from the old comment):

-         * ... They don't need to be left in place as LP_DEAD items
until VACUUM gets
-         * around to doing index vacuuming.

I don't feel like it adds a lot, but it is absent from the new
comment, so thought I would check.

> BTW:
>
> +                        * LP_REDIRECT, or LP_DEAD items to LP_UNUSED
> during pruning. We
> +                        * can't check much here except that, if the
> item is LP_NORMAL, it
> +                        * should have storage before it is set LP_UNUSED.
>
> Is it really helpful to check this here, or just confusing/grasping at
> straws? I mean, the requirement that LP_NORMAL items have storage is a
> general one, IIUC, not something that's specific to this situation. It
> feels like the equivalent of checking that your children don't set
> fire to the couch on Tuesdays.

Hmm. Yes. I suppose I was trying to find something to validate. Is it
worth checking that the line pointer is not already LP_UNUSED? Or is
that a bit ridiculous?

- Melanie