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

Chao Li <li.evan.chao@gmail.com>

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From: Chao Li <li.evan.chao@gmail.com>
To: Melanie Plageman <melanieplageman@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: 2026-03-04T08:59:41Z
Lists: pgsql-hackers 

> On Mar 3, 2026, at 23:52, Melanie Plageman <melanieplageman@gmail.com> wrote:
> 
> 
>> 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 understood that TransactionIdPrecedesOrEquals(InvalidTransactionId, prstate->cutoffs->OldestXmin) is true, but that would leave an impression to code readers that prstate->pagefrz.FreezePageConflictXid could not be InvalidTransactionId. Thus I think my version explicitly tells that prstate->pagefrz.FreezePageConflictXid could be InvalidTransactionId at the point.


>> I will continue with 0005 tomorrow.
> 

4 - 0005
```
  * Caller must have pin on the buffer, and must *not* have a lock on it.
  */
 void
-heap_page_prune_opt(Relation relation, Buffer buffer)
+heap_page_prune_opt(Relation relation, Buffer buffer, Buffer *vmbuffer)
```

I don’t see why vmbuffer has to be of pointer type. Buffer type is underlying int, I checked the last commit, vmbuffer only passes in data into the function without passing out anything.

As we add the new parameter vmbuffer, though it’s not used in this commit, I think it’d be better to update the header commit to explain what this parameter will do.

5  - 0006
```
+ *
+ * heap_fix_vm_corruption() makes changes to the VM and, potentially, the heap
+ * page, but it does not need to be done in a critical section because
+ * clearing the VM is not WAL-logged.
+ */
+static void
+heap_fix_vm_corruption(PruneState *prstate, OffsetNumber offnum)
```

Nit: why the last paragraph of the header comments uses the function name instead of “this function”? Looks like a copy-pasto.

6 - 0006
```
+		if (prstate->lpdead_items > 0)
+		{
+			ereport(WARNING,
+					(errcode(ERRCODE_DATA_CORRUPTED),
+					 errmsg("LP_DEAD item found on page marked as all-visible"),
+					 errdetail("relation \"%s\", page %u, tuple %u",
+							   RelationGetRelationName(prstate->relation),
+							   prstate->block, offnum)));
+		}
+		else
+		{
+			ereport(WARNING,
+					(errcode(ERRCODE_DATA_CORRUPTED),
+					 errmsg("tuple not visible to all found on page marked as all-visible"),
+					 errdetail("relation \"%s\", page %u, tuple %u",
+							   RelationGetRelationName(prstate->relation),
+							   prstate->block, offnum)));
+		}
```

I recently just learned that a detail message should use complete sentences, and end each with a period, and capitalize the first word of sentences. See https://www.postgresql.org/docs/current/error-style-guide.html.

7 - 0006
```
+	else if (prstate->vmbits & VISIBILITYMAP_VALID_BITS)
+	{
+		/*
+		 * As of PostgreSQL 9.2, the visibility map bit should never be set if
+		 * the page-level bit is clear.  However, it's possible that the bit
+		 * got cleared after heap_vac_scan_next_block() was called, so we must
+		 * recheck with buffer lock before concluding that the VM is corrupt.
+		 */
+		ereport(WARNING,
+				(errcode(ERRCODE_DATA_CORRUPTED),
+				 errmsg("page %u in \"%s\" is not marked all-visible but visibility map bit is set",
+						prstate->block,
+						RelationGetRelationName(prstate->relation))));
+	}
```

The comment says “we must recheck with buffer lock before…”, but it only log a warning message. Is the comment stale?

8 - 0007
```
+static void
+heap_page_bypass_prune_freeze(PruneState *prstate, PruneFreezeResult *presult)
+{
+	OffsetNumber maxoff = PageGetMaxOffsetNumber(prstate->page);
+	Page		page = prstate->page;
+
+	Assert(prstate->vmbits & VISIBILITYMAP_ALL_FROZEN ||
+		   (prstate->vmbits & VISIBILITYMAP_ALL_VISIBLE &&
+			!prstate->attempt_freeze));
+
+	/* We'll fill in presult for the caller */
+	memset(presult, 0, sizeof(PruneFreezeResult));
+
+	/*
+	 * Since the page is all-visible, a count of the normal ItemIds on the
+	 * page should be sufficient for vacuum's live tuple count.
+	 */
+	for (OffsetNumber off = FirstOffsetNumber;
+		 off <= maxoff;
+		 off = OffsetNumberNext(off))
+	{
+		if (ItemIdIsNormal(PageGetItemId(page, off)))
+			prstate->live_tuples++;
+	}
+
+	presult->live_tuples = prstate->live_tuples;
+
+	/* Clear any stale prune hint */
+	if (TransactionIdIsValid(PageGetPruneXid(page)))
+	{
+		PageClearPrunable(page);
+		MarkBufferDirtyHint(prstate->buffer, true);
+	}
+
+	presult->vmbits = prstate->vmbits;
+
+	if (!PageIsEmpty(page))
+		presult->hastup = true;
+}
```

* Given this function has done PageIsEmpty(page), that that is true, we don’t need to count live_tuples, right? That could be a tiny optimization.
* I see heap_page_bypass_prune_freeze() is only called in one place and immediately after prune_freeze_setup() and heap_fix_vm_corruption(), so prstate->vmbits must be 0, so do we need to do presult->vmbits = prstate->vmbits;?
* Do we need to set all_visible and all_frozen to presult?

0008 LGTM

I will continue with 0009 tomorrow.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/

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