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

Melanie Plageman <melanieplageman@gmail.com>

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From: Melanie Plageman <melanieplageman@gmail.com>
To: Chao Li <li.evan.chao@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Andrey Borodin <x4mmm@yandex-team.ru>, Kirill Reshke <reshkekirill@gmail.com>, Xuneng Zhou <xunengzhou@gmail.com>, Robert Haas <robertmhaas@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Heikki Linnakangas <hlinnaka@iki.fi>
Date: 2026-03-03T15:52:34Z
Lists: pgsql-hackers 
On Tue, Mar 3, 2026 at 2:33 AM Chao Li <li.evan.chao@gmail.com> wrote:
>
> 2 - 0003 - Does it make sense to also do the same renaming in PruneFreezeResult?

I could do that. Later commits remove them, so I thought it didn't
make sense. If only this commit goes in though, it would make sense.

> -                * Calculate what the snapshot conflict horizon should be for a record
> -                * freezing tuples. We can use the visibility_cutoff_xid as our cutoff
> -                * for conflicts when the whole page is eligible to become all-frozen
> -                * in the VM once we're done with it. Otherwise, we generate a
> -                * conservative cutoff by stepping back from OldestXmin.
> -                */
> -               if (prstate->set_all_frozen)
> -                       prstate->frz_conflict_horizon = prstate->visibility_cutoff_xid;
> -               else
> -               {
> -                       /* Avoids false conflicts when hot_standby_feedback in use */
> -                       prstate->frz_conflict_horizon = prstate->cutoffs->OldestXmin;
> -                       TransactionIdRetreat(prstate->frz_conflict_horizon);
> -               }
> +               Assert(TransactionIdPrecedesOrEquals(prstate->pagefrz.FreezePageConflictXid,
> +                                                                                        prstate->cutoffs->OldestXmin));
> ```
>
> At this point of Assert, can prstate->pagefrz.FreezePageConflictXid be InvalidTransactionId? My understanding is no, in that case, would it make sense to also Assert(prstate->pagefrz.FreezePageConflictXid != InvalidTransactionId)?

I think it is possible if we are doing some kind of freezing to a
multixact that we reach here and FreezePageConflictXid is
InvalidTransactionId.

> Otherwise, if prstate->pagefrz.FreezePageConflictXid is still possibly be InvalidTransactionId, then the Assert should be changed to something like:
>
> Assert(prstate->pagefrz.FreezePageConflictXid == InvalidTransactionId ||
>   TransactionIdPrecedesOrEquals(prstate->pagefrz.FreezePageConflictXid, prstate->cutoffs->OldestXmin)

This is covered by TransactionIdPrecedesOrEquals because
InvalidTransactionId is 0. We assume that in many places throughout
the code.

> I will continue with 0005 tomorrow.

Thanks for the review!

I noticed a serious bug in v35-0017: I pass hscan->modifies_base_rel
to heap_page_prune_opt() as rel_read_only, which is the opposite of
what I want to do -- it should be !hscan->modifies_base_rel. I'm going
to wait to fix it though and post a new v36 once I've batched up more
fixups.

- 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