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

• 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?: 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:

<class 'asyncpg.exceptions.InvalidColumnReferenceError'>: 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.

Riproduci il problema usando asyncpg 0.29.0, PostgreSQL 14.9, Python 3.11.6, SQLAlchemy 2.0.28 e il modello di connessione FastAPI segnalato con asyncio e uvloop. Confronta le prime cinque query con la sesta e documenta una riproduzione minima che mostri lo schema, l’istruzione ON CONFLICT generata e il comportamento della connessione.