Error: there is no unique or exclusion constraint matching the ON CONFLICT specification

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* **asyncpg version**: 0.29.0
* **PostgreSQL version**: 14.9
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: no
* **Python version**: 3.11.6
* **Platform**: macos, arm, m1 pro
* **Do you use pgbouncer?**: no
* **Did you install asyncpg with pip?**: yes
* **If you built asyncpg locally, which version of Cython did you use?**: no
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: yes

Hello everyone. So, I've written a fairly simple code that updates a table based on a composite index. I'm using sqlalchemy 2.0.28. The query is generated correctly, but exactly on the 6th query to the database, asyncpg supposedly throws an error:

`: there is no unique or exclusion constraint matching the ON CONFLICT specification`

Although, as I mentioned - the first 5 queries work fine. Reloading the application (fastapi) or rather creating a new connection, everything starts working again for the first 5 queries. I looked for the reason in sqlalchemy since there pool_size = 5. Changing this value still gives me the same error.

Reproduziere das Problem mit asyncpg 0.29.0, PostgreSQL 14.9, Python 3.11.6, SQLAlchemy 2.0.28 und dem gemeldeten FastAPI-Verbindungsmuster unter asyncio und uvloop. Vergleiche die ersten fünf Abfragen mit der sechsten und dokumentiere eine minimale Reproduktion, die das Schema, die generierte ON CONFLICT-Anweisung und das Verbindungsverhalten zeigt.

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