Re: support ALTER TABLE DROP EXPRESSION for virtual generated column

On Wednesday, March 26, 2025, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> jian he <jian(dot)universality(at)gmail(dot)com> writes:
> > the attached patch is to implement $subject.
>
> Why would this be a good idea? I don't see any principled fallback
> definition of the column. (No, "NULL" is not that.) Certainly we
> should support ALTER TABLE DROP COLUMN, but removing the expression
> and not providing a substitute seems semantically nonsensical.
>

I don’t follow how NULL got involved in this discussion. The proposal is
basically: turn the virtual expression into an equivalent stored
expression, then drop the expression.

I suppose it would make sense to first add an alter table command to allow
the user to do a virtual/stored mode swap manually before adding this,
which then just becomes a convenient way to specify swap-and-drop as a
single command.

David J.