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Subject: Re: Wrong comment for ReplicationSlotCreate
From: Chao Li <li.evan.chao@gmail.com>
In-Reply-To: 
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Date: Wed, 31 Dec 2025 10:32:17 +0800
Cc: Postgres hackers <pgsql-hackers@lists.postgresql.org>
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To: Daniil Davydov <3danissimo@gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/AC737D02-4021-4800-9F2B-5D93239BD043%40gmail.com>
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> On Dec 30, 2025, at 18:07, Daniil Davydov <3danissimo@gmail.com> =
wrote:
>=20
> Hi,
>=20
> On Tue, Dec 30, 2025 at 4:18=E2=80=AFPM Chao Li =
<li.evan.chao@gmail.com> wrote:
>>=20
>> But I think the updated version is too simple. It loses the =
information that enabling two_phase later can result in missing PREPARE.
>>=20
>> So, I would suggest something like:
>> ```
>> * two_phase: If enabled, allows decoding of prepared transactions.
>> *     Note that enabling this option after decoding has already =
advanced
>> *     may result in missing PREPARE records for transactions that =
were
>> *     prepared before the option was enabled.
>>=20
>> ```
>=20
> Thanks for the review!
>=20
> As far as I understand, if the publisher prepares a transaction and
> then subscriber
> tries to create a subscription, walsender will wait until the prepared
> transaction is
> finished (during execution of  CREATE_REPLICATION_SLOT command).
> We can find this logic inside the SnapBuildFindSnapshot function. =
Thus, we
> cannot miss any PREPARE record for the created slot.
>=20
> Am I missing something?
>=20
> --
> Best regards,
> Daniil Davydov


You=E2=80=99re right that during CREATE_REPLICATION_SLOT, =
SnapBuildFindSnapshot() ensures we don=E2=80=99t miss PREPARE records =
for prepared transactions that exist at creation time.

1462aad2e introduced support for altering logical replication slot =
options, including two_phase, after the slot has been created. The =
commit comment says:
```
    Changing the 'two_phase' option for a subscription from 'true' to =
'false'
    is permitted only when there are no pending prepared transactions
    corresponding to that subscription. Otherwise, the changes of =
already
    prepared transactions can be replicated again along with their =
corresponding
    commit leading to duplicate data or errors.
```

true->false is not allowed, however false->true is permitted. So, I =
think the old comment is still possible today:
```
*     Note that enabling this option after decoding has already advanced
*     may result in missing PREPARE records for transactions that were
*     prepared before the option was enabled.
```

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