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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: <81DAFACF-7D55-4A84-ACB0-0425D1669DB4@gmail.com>
Date: Fri, 6 Mar 2026 10:40:53 +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 5, 2026, at 16:52, Chao Li <li.evan.chao@gmail.com> wrote:
>=20
>=20
>=20
>> 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
>=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
>=20
> Nit: prstate.all_frozen -> prstate.set_all_frozen
>=20
> I saw you have fixed this in 0010, but I think it=E2=80=99s better =
also fix it here.
>=20
> 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;
> ```
>=20
> 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.
>=20
> 11 - 0010
> ```
> + BlockNumber new_all_visible_pages;
> + BlockNumber new_all_visible_frozen_pages;
> + BlockNumber new_all_frozen_pages;
> ```
>=20
> 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.
>=20
> 12 - 0010
> ```
> +  * conflict would ahve been handled in reaction to the WAL record =
freezing
> ```
>=20
> Nit: ahve -> have
>=20
> 0011 LGTM
>=20
> 13 - 0012 - bufmask.c
> ```
> +  * we don't mark the page all-visible. See =
heap_xlog_prune_and_freeze()
> +  * for more details.
> ```
>=20
> I don=E2=80=99t find a function named heap_xlog_prune_and_freeze().
>=20
> 14 - 0012 - heapam_xlog.c
> ```
> +  * same approach is taken when replaying XLOG_HEAP2_PRUNE* records =
(see
> +  * heap_xlog_prune_and_freeze()).
> ```
>=20
> Same as 13.
>=20
> 0013 LGTM
>=20
> I will try to finish the rest 5 commits tomorrow.
>=20

15 - 0014 - execMain.c
```
@@ -3027,6 +3035,7 @@ EvalPlanQualStart(EPQState *epqstate, Plan =
*planTree)
 	rcestate->es_range_table_size =3D =
parentestate->es_range_table_size;
 	rcestate->es_relations =3D parentestate->es_relations;
 	rcestate->es_rowmarks =3D parentestate->es_rowmarks;
+	rcestate->es_modified_relids =3D =
parentestate->es_modified_relids;
```

Here it just assigns the BMS pointer to rcestate->es_modified_relids. I =
am not sure if further bms_add_member() will still happen, if yes, it =
might be safer to do bms_copy(parentestate->es_modified_relids), because =
a further bms_add_member() may cause a new memory allocated and the old =
pointer stale.

16 - 0014 - execUtils.c
```
for (rti =3D 1; rti <=3D estate->es_range_table_size; rti++)
```

Nit: I have seen several recent commits that performed cleanups to =
switch to use for loop var like:
```
for (Index rti =3D 1; rti <=3D estate->es_range_table_size; rti++)
```

17 - 0015

The commit message subject line says =E2=80=9CMake begin_scan() =
functions take a flags argument=E2=80=9D, where begin_scan() seems =
inaccurate, for example, table_index_fetch_begin() is not =E2=80=9Cbegin =
scan=E2=80=9D.

Otherwise 0015 LGTM.

18 - 0016 - tableam.h
```
 	/* unregister snapshot at scan end? */
 	SO_TEMP_SNAPSHOT =3D 1 << 9,
+	/* set if the query doesn't modify the relation */
+	SO_HINT_REL_READ_ONLY =3D 1 << 10,
 }			ScanOptions;
```

Nit: maybe add an empty line before the new flag.

19 - 0017 - heapam_handler.c
```
@@ -147,7 +147,8 @@ heapam_index_fetch_tuple(struct IndexFetchTableData =
*scan,
 		 */
 		if (prev_buf !=3D hscan->xs_cbuf)
 			heap_page_prune_opt(hscan->xs_base.rel, =
hscan->xs_cbuf,
-								=
&hscan->xs_vmbuffer);
+								=
&hscan->xs_vmbuffer,
+								=
hscan->modifies_base_rel);
```

This feels like a bug. heap_page_prune_opt takes the first parameter =
rel_read_only, but hscan->modifies_base_rel means not read-only, so here =
we should use =E2=80=9C!hscan->modifies_base_rel=E2=80=9D.

Oh, when I read back your previous email, you have found this bug.

20 - 0018
In heap_insert(), you do:
```
+	if (TransactionIdIsNormal(xid) && !(options & =
HEAP_INSERT_FROZEN))
+		PageSetPrunable(page, xid);
```

But in heap_multi_insert(), you do:
```
+		if (!all_frozen_set && TransactionIdIsNormal(xid))
+			PageSetPrunable(page, xid);
```

Is the option check " !(options & HEAP_INSERT_FROZEN))=E2=80=9D also =
needed by heap_multi_insert?

~~ Done of this round review ~~

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