Re: BUG #18970: Atempt to alter type of table column used in row type with check leads to assertion failure

jian he <jian.universality@gmail.com> writes:
> SET EXPRESSION both can have this issue.
> so i also add more test cases for stored/virtual generated columns.

This patch is a lot larger than I was expecting, and I think it's
misguided.  You argue that

+     * Changing a virtual generated column's expression is akin to altering its
+     * type, requiring a call to find_composite_type_dependencies to check if
+     * the virtual generated column is used in any table.
+     * Therefore we need add this defval to tab->newvals for virtual generated
+     * column too, so Phase3 will call find_composite_type_dependencies.

but I think that is in fact wrong.  The implementation restriction
we have is that we lack code to run around and physically change
the stored values of columns that are not top-level table columns.
However, in the case of a virtual column we don't need to change
anything about the storage: once we've fixed the catalog metadata,
we're done.  So I'm not seeing the need to add all this stuff
other than the additional locking calls.

            regards, tom lane