Re: Why cannot alter a column's type when it's used by a generated column - Mailing list pgsql-hackers

Hi Jian,

Thanks for looking into this problem.

 

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,

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