cursor prefetch with bounded lag

还没有人认领这个 Issue。

EDIT: nvm, looks like there's really no mechanism in the pgwire protocol that would support this behavior.

Would it at all be possible to support prefetching a batch of up to N rows via a cursor, as compared to waiting for exactly N rows to be available?

In other words, I would like to have a `max_lag` parameter similar to `timeout`, but with different semantics. If after `max_lag` at least 1 row, yet less than `prefetch` rows are available, do return the available rows.

Current api is:

```
cursor(query, *args, prefetch=None, timeout=None, record_class=None)
```

I would like it to be augmented as:

```
cursor(query, *args, prefetch=None, max_lag=None, timeout=None, record_class=None)
```

Is there any intrinsic limitation that would make this not worth the effort or simply there has been no interest for anything like that?

Thank you!

首先验证 issue 中所述的 pgwire 限制，并审查报告中描述的现有游标 API。未指定任何文件或测试，并且所请求的行为可能无法实现；要完成此项工作，需要确认一种受协议支持的设计以及相应的项目范围。

由索引模型根据 Issue 内容生成。