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

Alexey Makhmutov <a.makhmutov@postgrespro.ru>

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From: Alexey Makhmutov <a.makhmutov@postgrespro.ru>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Andres Freund <andres@anarazel.de>, Kirill Reshke <reshkekirill@gmail.com>, Andrey Borodin <x4mmm@yandex-team.ru>, Robert Haas <robertmhaas@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>
Date: 2026-04-06T14:30:51Z
Lists: pgsql-hackers 
Hi Melanie,

Sorry for the late note for the already committed patch, but I have a 
question on the last part of the 'heap_xlog_prune_freeze' function 
related to the FSM update (it was committed in add323d -'Eliminate 
XLOG_HEAP2_VISIBLE from vacuum phase III').

Currently it contains the following logic:
	
...
Size	freespace = 0;
...
if (BufferIsValid(buffer))
{
	if ((xlrec.flags & (XLHP_HAS_REDIRECTIONS |
						XLHP_HAS_DEAD_ITEMS |
						XLHP_HAS_NOW_UNUSED_ITEMS)) ||
		(vmflags & VISIBILITYMAP_VALID_BITS))
		freespace = PageGetHeapFreeSpace(BufferGetPage(buffer));
...
	UnlockReleaseBuffer(buffer);
}
...
if (freespace > 0)
	XLogRecordPageWithFreeSpace(rlocator, blkno, freespace);
...
		
My question is about the last check ('freespace > 0') - do we really 
want to call 'XLogRecordPageWithFreeSpace' only if 'freespace' is 
greater than 0? As I understand, the zero value is a perfectly valid 
output of the 'PageGetHeapFreeSpace' call (i.e. page has no space or no 
free line items while we mark all rows as frozen/visible), but with the 
current implementation we will skip FSM update in such case. Maybe we 
need to use additional flag (i.e. 'need_fsm_update'), set it before 
calling 'PageGetHeapFreeSpace' and then check before the 
'XLogRecordPageWithFreeSpace' invocation?

Thanks,
Alexey

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