Re: Document atthasmissing default optimization avoids verification table scan

"David G. Johnston" <david.g.johnston@gmail.com> writes:
> Or, we can leave it where things are and make sure the reader understands
> there are two paths to having a NOT NULL constraint on the newly added
> column.  Something like:

> "If you plan on having a NOT NULL constraint on the newly added column you
> should add it as a column constraint during the ADD COLUMN command.  If you
> add it later via ALTER COLUMN SET NOT NULL the table will have to be
> completely scanned in order to ensure that no null values were inserted."

The first way also requires having a non-null DEFAULT, of course, and
then also that default value must be a constant (else you end up with
a table rewrite which is even worse).  This sort of interaction
between features is why I feel that a separate unified discussion
is the only reasonable solution.

            regards, tom lane