Horton

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

Melanie Plageman <melanieplageman@gmail.com>

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From: Melanie Plageman <melanieplageman@gmail.com>
To: Pg Hackers <pgsql-hackers@postgresql.org>
Cc: Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>, Peter Geoghegan <pg@bowt.ie>, Michael Paquier <michael@paquier.xyz>
Date: 2023-12-21T21:36:12Z
Lists: pgsql-hackers 
On Fri, Nov 17, 2023 at 6:12 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> On Mon, Nov 13, 2023 at 5:28 PM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
> > When there are no indexes on the relation, we can set would-be dead
> > items LP_UNUSED and remove them during pruning. This saves us a vacuum
> > WAL record, reducing WAL volume (and time spent writing and syncing
> > WAL).
> ...
> > Note that (on principle) this patch set is on top of the bug fix I
> > proposed in [1].
> >
> > [1] https://www.postgresql.org/message-id/CAAKRu_YiL%3D44GvGnt1dpYouDSSoV7wzxVoXs8m3p311rp-TVQQ%40mail.gmail.com
>
> Rebased on top of fix in b2e237afddc56a and registered for the january fest
> https://commitfest.postgresql.org/46/4665/

I got an off-list question about whether or not this codepath is
exercised in existing regression tests. It is -- vacuum.sql tests
include those which vacuum a table with no indexes and tuples that can
be deleted.

I also looked through [1] to see if there were any user-facing docs
which happened to mention the exact implementation details of how and
when tuples are deleted by vacuum. I didn't see anything like that, so
I don't think there are user-facing docs which need updating.

- Melanie

[1] https://www.postgresql.org/docs/devel/routine-vacuuming.html

Commits

  1. Combine FSM updates for prune and no-prune cases.

  2. Remove LVPagePruneState.

  3. Move VM update code from lazy_scan_heap() to lazy_scan_prune().

  4. Optimize vacuuming of relations with no indexes.

  5. Be more consistent about whether to update the FSM while vacuuming.

  6. Remove hastup from LVPagePruneState.

  7. Use scanned_pages to decide when to failsafe check.

  8. Simplify lazy_scan_heap's handling of scanned pages.

  9. While vacuuming a large table, update upper-level FSM data every so often.