sql: adding a computed column doesn't validate against existing values

[fqazi]

Describe the problem

Right now it's possible to add computed columns where the generated values for a subset of columns
can never be properly computed. For example a computed column could be the division between two
columns, in a given table: ALTER TABLE ADD COLUMN c INT AS c1/c2 VIRTUAL. If there is any c2 that is
zero then this computed column would have a division by zero. When the computed column is STORED this
statement will rollback. But, when its virtual this column will be added and any SELECT involving the problematic
column will fail.

The ideal solution here is to have validation for VIRTUAL computed columns, where when a column is
added the existing values in the table are validated for the generated expression. If any of the expressions
can't be generated then the error is surfaced similar to a STORED computed column.

Jira issue: CRDB-19740

[ajwerner]

@mgartner I think you'll find this one to be interesting. I think we should just add a validation step.

[ajwerner]

Is this a duplicate of #81698?

[postamar]

It seems so. Let's do this only in the declarative schema changer.