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

还没有人认领这个 Issue。

* **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.

使用 asyncpg 0.29.0、PostgreSQL 14.9、Python 3.11.6、SQLAlchemy 2.0.28，以及在 asyncio 和 uvloop 下报告的 FastAPI 连接模式来复现此问题。将前五个查询与第六个查询进行比较，并记录一个展示 schema、生成的 ON CONFLICT 语句和连接行为的最小复现。

由索引模型根据 Issue 内容生成。