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

Nobody has claimed this yet.

* **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.

Reproduce the issue using asyncpg 0.29.0, PostgreSQL 14.9, Python 3.11.6, SQLAlchemy 2.0.28, and the reported FastAPI connection pattern under asyncio and uvloop. Compare the first five queries with the sixth and document a minimal reproduction showing the schema, generated ON CONFLICT statement, and connection behavior.

Written by the indexing model from the issue text.