Re: Why cannot alter a column's type when it's used by a generated column

Hi Jian,

Thanks for looking into this problem.

> The attached patch removes this restriction.
> it need more polish, but it's good enough to use it to verify the bug I
> reported
> on
>
> https://postgr.es/m/CACJufxHZsgn3zM5g-x7YmtFGzNDnRwR07S+GYfiUs+tZ45MDDw@mail.gmail.com

I think we can keep the discussion in your thread. Actually, I got this
idea from your discussion.

But for this topic, I don't want to go that far in the first place, because
completely allowing that might require more discussion.

Here I am just proposing a patch for a small step. When any dependent
generated column has SET EXPRESSION, then we can allow the column type
change:

```
-- Before this patch, the recommended usage was:
ALTER TABLE gtest
DROP COLUMN x,
ALTER COLUMN a TYPE float8,
ADD COLUMN x bigint GENERATED ALWAYS AS ((a::int + b) * 2) STORED;

-- With this patch, the statement is simplified as:
ALTER TABLE gtest
ALTER COLUMN a TYPE float8,
ALTER COLUMN x SET EXPRESSION ((a::int + b) * 2);
```

Best regards,
Chao Li (Evan)
---------------------
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