Does COPY FROM gaurentee no partial rows?

还没有人认领这个 Issue。

A quick question, since this is not very obvious from the postgres or asyncpg docs: under what circumstances, if any, does `copy_from_query` guarantee that no partial rows will be delivered to the `output` coroutine?

In my experience this hasn't happened, and I could try some more exhaustive testing, but I imagine/hope that someone might just already know the answer.

To me the options seem to be:
- If the networking code reads one line at a time, then the 'text' format guarantees full rows, but not necessarily 'csv' or 'binary'
- Asyncpg itself or postgres itself may explicitly implement such a guarantee?
- Otherwise, there is no guarantee.

先从 PostgreSQL 和 asyncpg 关于 copy_from_query 的文档开始，然后追踪现有 asyncpg 材料中对其输出协程的描述。记录文本、CSV 和二进制输出在什么情况下（如果存在）可以避免部分行，并向用户明确说明这一保证。

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