Re: support ALTER COLUMN SET EXPRESSION over virtual generated column with check constraint

On Thu, Jan 15, 2026 at 11:29 PM Matheus Alcantara
<matheusssilv97(at)gmail(dot)com> wrote:
>
> + /*
> + * Find everything that depends on the column (constraints, indexes, etc),
> + * and record enough information to let us recreate the objects after
> + * rewrite.
> + */
> + RememberAllDependentForRebuilding(tab, AT_SetExpression, rel, attnum, colName);
> +
> Perhaps this comments should be updated since we are now collecting
> these dependencies for virtual generated columns too that it not require
> a table rewrite.
>
> ---

I guess we can change it as:

/*
* Find everything that depends on the column (constraints, indexes, etc),
* and record enough information to let us recreate the objects.
*/
RememberAllDependentForRebuilding(tab, AT_SetExpression, rel,
attnum, colName);

>
> I think that it would be good to update the SET EXPRESSION AS
> documentation on doc/src/sgml/ref/alter_table.sgml to mention that for
> virtual columns the table is not rewritten but a full table scan may
> still be needed if the column has check constraints.
>
> --
> Matheus Alcantara
> EDB: https://www.enterprisedb.com

In doc/src/sgml/ref/alter_table.sgml, I intended to change it as follows:

<varlistentry id="sql-altertable-desc-set-expression">
<term><literal>SET EXPRESSION AS</literal></term>
<listitem>
<para>
This form replaces the expression of a generated column. Existing data
in a stored generated column is rewritten and all the future changes
will apply the new generation expression.
+ Virtual generated columns do not require a table rewrite.
+ However if the column is part of a <literal>CHECK</literal> constraint
+ expression, the constraint will be rebuilt, requiring a table scan to
+ ensure that existing rows meet the constraint.
</para>