From: Chao Li <li.evan.chao@gmail.com>
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Subject: Question: should we block loopback logical replication to the same
 database?
Message-Id: <25C3F6A7-DC52-48AE-B43C-0D8665CAB74A@gmail.com>
Date: Tue, 7 Apr 2026 16:26:26 +0800
To: PostgreSQL-development <pgsql-hackers@postgresql.org>
Archived-At: 
 <https://www.postgresql.org/message-id/25C3F6A7-DC52-48AE-B43C-0D8665CAB74A%40gmail.com>
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Hi,

While testing a patch today, I tried to set up logical replication on =
the same cluster. Although I have done that many times before, I made a =
mistake this time: I accidentally ran the CREATE SUBSCRIPTION command in =
the source database session, and the subscription was created =
successfully:
```
evantest=3D# CREATE SUBSCRIPTION mysub CONNECTION 'host=3Dlocalhost =
dbname=3Devantest user=3Drepl password=3Dsecret' PUBLICATION mypub WITH =
(create_slot =3D false);
CREATE SUBSCRIPTION
```

After that, something weird started to happen. The destination table was =
empty, but the logs kept reporting duplicate-key errors. It took me a =
while to realize that the problem was this mistake.

This made me wonder whether we should block this kind of loopback =
logical replication to the same database. I am not aware of a way to use =
different destination table names, so this setup does not seem useful. =
Blocking it could help prevent this kind of unnecessary mistake.

Before working on a patch, I wanted to check with hackers first. If =
there are no objections, I can propose a patch. Please let me know if I =
am missing something.

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