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

> On Mar 3, 2026, at 23:52, Melanie Plageman <melanieplageman(at)gmail(dot)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/