[Bug] Wrong number of columns Error

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* **asyncpg version**: 0.23
* **PostgreSQL version**: 13
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: Yandex Cloud
* **Python version**: 3.7
* **Platform**: Ubuntu 18.04
* **Do you use pgbouncer?**: yes
* **Did you install asyncpg with pip?**: no
* **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)?**: don't know

We have a web server on Python which is sending queries to the PostgreSQL cluster. After adding a column to the table using a migration without disconnecting I encounter errors with some queries to that table. Example of the query below:

`WITH row_info as ( SELECT id FROM UNNEST($1::scheme.affected_table[]) ) DELETE FROM scheme.affected_table AS entities USING row_info WHERE entities.id = row_info.id;`

To $1 I pass the list of dictionaries like `[{'id': 1, 'field1': 'value1', ...}]`.

Before the migration or from new connections the query works perfectly. But for queries from web-server I get "wrong number of columns: 11, expected 12". Rebooting the web-server resolves the problem.

I think the problem can be related to Prepared Statements or encoding of Mapping.

The report provides no source file or test entry point. Start by reproducing the UNNEST query after a schema migration with an existing connection, then compare it with a new connection while investigating prepared statements and mapping encoding. Done means the stale-connection query no longer reports a column-count mismatch.

Written by the indexing model from the issue text.