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Subject: Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM
 on-access)
From: Chao Li <li.evan.chao@gmail.com>
In-Reply-To: <C3AB3F5B-626E-4AAA-9529-23E9A20C727F@gmail.com>
Date: Thu, 5 Mar 2026 16:52:45 +0800
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>
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To: Melanie Plageman <melanieplageman@gmail.com>
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> On Mar 4, 2026, at 16:59, Chao Li <li.evan.chao@gmail.com> wrote:
>=20
>=20
>=20
>> On Mar 3, 2026, at 23:52, Melanie Plageman =
<melanieplageman@gmail.com> wrote:
>>=20
>>=20
>>> Otherwise, if prstate->pagefrz.FreezePageConflictXid is still =
possibly be InvalidTransactionId, then the Assert should be changed to =
something like:
>>>=20
>>> Assert(prstate->pagefrz.FreezePageConflictXid =3D=3D =
InvalidTransactionId ||
>>> =
TransactionIdPrecedesOrEquals(prstate->pagefrz.FreezePageConflictXid, =
prstate->cutoffs->OldestXmin)
>>=20
>> This is covered by TransactionIdPrecedesOrEquals because
>> InvalidTransactionId is 0. We assume that in many places throughout
>> the code.
>>=20
>=20
> 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.
>=20
>=20
>>> I will continue with 0005 tomorrow.
>>=20
>=20
> 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)
> ```
>=20
> I don=E2=80=99t 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.
>=20
> As we add the new parameter vmbuffer, though it=E2=80=99s not used in =
this commit, I think it=E2=80=99d be better to update the header commit =
to explain what this parameter will do.
>=20
> 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)
> ```
>=20
> Nit: why the last paragraph of the header comments uses the function =
name instead of =E2=80=9Cthis function=E2=80=9D? Looks like a =
copy-pasto.
>=20
> 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)));
> + }
> ```
>=20
> 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.
>=20
> 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))));
> + }
> ```
>=20
> The comment says =E2=80=9Cwe must recheck with buffer lock =
before=E2=80=A6=E2=80=9D, but it only log a warning message. Is the =
comment stale?
>=20
> 8 - 0007
> ```
> +static void
> +heap_page_bypass_prune_freeze(PruneState *prstate, PruneFreezeResult =
*presult)
> +{
> + OffsetNumber maxoff =3D PageGetMaxOffsetNumber(prstate->page);
> + Page page =3D 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 =3D FirstOffsetNumber;
> + off <=3D maxoff;
> + off =3D OffsetNumberNext(off))
> + {
> + if (ItemIdIsNormal(PageGetItemId(page, off)))
> + prstate->live_tuples++;
> + }
> +
> + presult->live_tuples =3D prstate->live_tuples;
> +
> + /* Clear any stale prune hint */
> + if (TransactionIdIsValid(PageGetPruneXid(page)))
> + {
> + PageClearPrunable(page);
> + MarkBufferDirtyHint(prstate->buffer, true);
> + }
> +
> + presult->vmbits =3D prstate->vmbits;
> +
> + if (!PageIsEmpty(page))
> + presult->hastup =3D true;
> +}
> ```
>=20
> * Given this function has done PageIsEmpty(page), that that is true, =
we don=E2=80=99t 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 =3D =
prstate->vmbits;?
> * Do we need to set all_visible and all_frozen to presult?
>=20
> 0008 LGTM
>=20
> I will continue with 0009 tomorrow.
>=20

9 - 0009
=C2=B7=C2=B7=C2=B7
+	 * Currently, only VACUUM performs freezing, but other callers =
may in the
+	 * future. Other callers must initialize prstate.all_frozen to =
false,
=C2=B7=C2=B7=C2=B7

Nit: prstate.all_frozen -> prstate.set_all_frozen

I saw you have fixed this in 0010, but I think it=E2=80=99s better also =
fix it here.

10 - 0010
```
+	 * Whether or not the page was newly set all-visible and =
all-frozen during
+	 * phase I of vacuuming.
 	 */
-	uint8		vmbits;
+	BlockNumber new_all_visible_pages;
+	BlockNumber new_all_visible_frozen_pages;
+	BlockNumber new_all_frozen_pages;
```

These 3 fields are actually counts rather than pointers to blocks, using =
type BlockNumber are quite confusing, though underlying BlockNumber is =
uint32. I think they can be just int type.

11 - 0010
```
+ BlockNumber new_all_visible_pages;
+ BlockNumber new_all_visible_frozen_pages;
+ BlockNumber new_all_frozen_pages;
```

I don=E2=80=99t see where these 3 fields are initialized. In =
lazy_scan_prune(), presult is defined as:
```
    PruneFreezeResult presult;
```
So, those fields will hold random values.

12 - 0010
```
+	 * conflict would ahve been handled in reaction to the WAL =
record freezing
```

Nit: ahve -> have

0011 LGTM

13 - 0012 - bufmask.c
```
+	 * we don't mark the page all-visible. See =
heap_xlog_prune_and_freeze()
+	 * for more details.
```

I don=E2=80=99t find a function named heap_xlog_prune_and_freeze().

14 - 0012 - heapam_xlog.c
```
+	 * same approach is taken when replaying XLOG_HEAP2_PRUNE* =
records (see
+	 * heap_xlog_prune_and_freeze()).
```

Same as 13.

0013 LGTM

I will try to finish the rest 5 commits tomorrow.

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