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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: 2026-03-11T17:01:36Z
Lists: pgsql-hackers

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On Fri, Mar 6, 2026 at 6:33 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> Thanks for the review! Attached is v36. I've pushed some of the early
> patches in the set and this is what is left.

I've gone ahead and pushed another of the introductory commits.
Attached v37 has the remaining patches.

The one change is that I've removed get_conflict_xid(). I determined
that in the current code that we cannot end up in the scenario where
we didn't prune or freeze and the page was already all-visible but not
all-frozen. The closest scenario would be one where the page was
all-frozen, we cleared the all-frozen bit because we did a SELECT FOR
UPDATE on one of the tuples, then vacuum freezes the page. Even though
we are just invalidating the xmax, it still counts as freezing.
However, in this case we will not advance the FreezePageConflictXid,
so the snapshot conflict horizon will still correctly be
InvalidTransactionId. I believe in all cases we will correctly set the
conflict horizon to InvalidTransactionId with this much simpler
conflict xid calculation:

    conflict_xid = InvalidTransactionId;
    if (do_set_vm)
        conflict_xid = prstate.newest_live_xid;
    if (do_freeze &&
TransactionIdFollows(prstate.pagefrz.FreezePageConflictXid,
conflict_xid))
        conflict_xid = prstate.pagefrz.FreezePageConflictXid;
    if (do_prune && TransactionIdFollows(prstate.latest_xid_removed,
conflict_xid))
        conflict_xid = prstate.latest_xid_removed;

The only outstanding question I have is about pd_prune_xid on insert:

I think we only want to set pd_prune_xid on insert if the transaction
ID is normal. Bootstrap mode does call heap_insert(), so we need to
check the xid before setting it. The only question is then if we want
the same guard on replay. Bootstrap mode won't actually insert a WAL
record, so we don't need this check I think. However, I think it is
better to have it for consistency with normal mode.

- 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