Horton
Menu

Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM on-access)

Melanie Plageman <melanieplageman@gmail.com>

View thread
From: Melanie Plageman <melanieplageman@gmail.com>
To: Tomas Vondra <tomas@vondra.me>
Cc: Andres Freund <andres@anarazel.de>, Kirill Reshke <reshkekirill@gmail.com>, Chao Li <li.evan.chao@gmail.com>, Andrey Borodin <x4mmm@yandex-team.ru>, Xuneng Zhou <xunengzhou@gmail.com>, Robert Haas <robertmhaas@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Heikki Linnakangas <hlinnaka@iki.fi>
Date: 2026-03-27T19:31:04Z
Lists: pgsql-hackers 
On Thu, Mar 26, 2026 at 12:07 PM Tomas Vondra <tomas@vondra.me> wrote:
>
> >> Ah, so we expect people to invent their "own" flags, outside what's in
> >> ScanOptions? Or do I misunderstand how it works? (I admit not reading
> >> the whole massive thread, as I was only interested in using the flags in
> >> my own patch.)
> >
> > Yes, this isn't really explored in the rest of the thread. I thought
> > since the flags are threaded all the way through and they can
> > set/check the flags in the table AM-specific layer, it would make
> > sense that they could choose flags for their own purposes. They don't
> > have to wait for consensus on getting a new SO type added. I don't
> > know if this is a bad idea. However, changing the table AM wrappers
> > seems more justifiable if we are making them extensible in this way.
> >
>
> No idea. Do we have an example of a TAM actually needing this? If not,
> I'd probably advise to remove that and keep the patch simpler. My past
> attempts to future-proof a patch like this rarely worked.

Yea, not allowing that doesn't really simplify the patch.
But, talking to Andres off-list yesterday, he reminded me that users
can simply add a new member to their table access method-specific scan
descriptor (e.g. HeapScanDescData could get a new member). The value
of flags lies in enabling table AM-agnostic executor code to pass
flags through the table AM to the scan code. Besides my read-only hint
scan option, he gave some examples -- like a hint to the scan that
there is a LIMIT on the query. I think that is compelling.

While exploring this, I realized that for a few internal flags, such
as SO_ALLOW_STRAT and SO_ALLOW_SYNC, we have table scan functions,
like table_beginscan_strat(), that accept parameters for setting those
flags. They are basically the same as table_beginscan() but give users
control over those flags. I think we can use the flags parameter to
deprecate some of these specialized table scan functions. I think we
can simplify the scan_rescan() callback as well. I don't think it
makes sense to do it this late in the 19 release, though. All of those
changes require having a flags parameter in the top level scan
wrappers first. So, I think it is reasonable to do just that this
release.

- Melanie

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Stabilize btree_gist test against on-access VM setting

  2. Stabilize plancache test against on-access VM setting

  3. Make local buffers pin limit more conservative

  4. Update FSM during prune/freeze replay even if freespace is zero

  5. Pass down information on table modification to scan nodes

  6. Thread flags through begin-scan APIs

  7. Remove PlannedStmt->resultRelations in favor of resultRelationRelids

  8. Make it cheap to check if a relation is modified by a query

  9. Keep newest live XID up-to-date even if page not all-visible

  10. Add pruning fast path for all-visible and all-frozen pages

  11. Detect and fix visibility map corruption in more cases

  12. Save vmbuffer in heap-specific scan descriptors for on-access pruning

  13. Avoid BufferGetPage() calls in heap_update()

  14. Initialize missing fields in CreateExecutorState()

  15. Prefix PruneState->all_{visible,frozen} with set_

  16. Add PageGetPruneXid() helper

  17. Move commonly used context into PruneState and simplify helpers

  18. Remove table_scan_analyze_next_tuple unneeded parameter OldestXmin

  19. Simplify visibility check in heap_page_would_be_all_visible()

  20. Rename LVRelState VM-related logging counters

  21. Fix flakiness in the pg_visibility VM-only vacuum test by using a temporary table.

  22. Refactor lazy_scan_prune() VM clear logic into helper

  23. Eliminate use of cached VM value in lazy_scan_prune()

  24. Combine visibilitymap_set() cases in lazy_scan_prune()

  25. Fix const qualification in prune_freeze_setup()

  26. Simplify vacuum visibility assertion

  27. Split heap_page_prune_and_freeze() into helpers

  28. Assert that cutoffs are provided if freezing will be attempted

  29. Split PruneFreezeParams initializers to one field per line

  30. Refactor heap_page_prune_and_freeze() parameters into a struct

  31. Make heap_page_is_all_visible independent of LVRelState

  32. Inline TransactionIdFollows/Precedes[OrEquals]()

  33. Add helper for freeze determination to heap_page_prune_and_freeze

  34. Bump XLOG_PAGE_MAGIC after xl_heap_prune change

  35. Correct prune WAL record opcode name in comment

  36. Add error codes when vacuum discovers VM corruption

  37. Remove unused xl_heap_prune member, reason

  38. Remove unneeded VM pin from VM replay

  39. Add assert and log message to visibilitymap_set

  40. Stamp HEAD as 19devel.

  41. Add error codes to some corruption log messages