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

Tomas Vondra <tomas@vondra.me>

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From: Tomas Vondra <tomas@vondra.me>
To: Melanie Plageman <melanieplageman@gmail.com>
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: 2026-03-26T16:07:41Z
Lists: pgsql-hackers 
On 3/26/26 15:51, Melanie Plageman wrote:
> On Wed, Mar 25, 2026 at 7:29 PM Tomas Vondra <tomas@vondra.me> wrote:
>>
>>>> - Do we really want to pass two sets of flags to table_beginscan_common?
>>>>  I realize it's done to ensure "users" don't use internal flags, but
>>>> then maybe it'd be better to do that check in the places calling the
>>>> _common? Someone adding a new caller can break this in various ways
>>>> anyway, e.g. by setting bits in the internal flags, no?
>>>
>>> Yes, callers of table_beginscan_common() could pass flags they
>>> shouldn't in internal_flags. But I was mostly trying to prevent the
>>> case where a user picks a flag that overlaps with an internal flag,
>>> conditionally passes it as a user flag, and then when they test for it
>>> in their AM-specific code, they aren't actually checking if their own
>>> flag is set.
>>
>> 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.

If we want to give TAMs the opportunity to define custom flags, do we
already do something like that elsewhere? Is there a precedent how to do
that? If we allow the TAM to pick arbitrary flag values, it's easy to
end up with collisions later (if we add a new internal flag). Maybe
there is a way to prevent that? E.g. we could restrict internal flags to
0x0000FFFF, and custom flags to 0xFFFF0000?

regards

-- 
Tomas Vondra

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