Re: Fast AT ADD COLUMN with DEFAULTs

On 2016-10-05 18:58:47 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > On 2016-10-05 15:44:56 -0700, Jeff Janes wrote:
> >>> No, "a second “exist default"" was mentioned, i.e. it is an additional
> >>> column in a system table (pg_attribute) as default column values of
> >>> the "pre-alter" era. It solves changing of the default expression of
> >>> the same column later.
> 
> > Don't think that actually solves the issue. The default might be unset
> > for a while, for example. Essentially you'd need to be able to associate
> > arbitrary number of default values with an arbitrary set of rows.
> 
> I think it does work, as long as the "exists default" is immutable.
> (For safety, personally, I'd restrict it to be a verbatim constant.)
> The point is that you apply that when you are reading a row that has
> so few columns that it must predate the original ALTER TABLE ADD COLUMN.
> Subsequent ADD/DROP/SET DEFAULT affect the "active default" and hence
> insertions that happen after them, but they don't affect the
> interpretation of old rows.  And of course all rows inserted after the
> ADD COLUMN contain explicit values of the column, so their meaning is
> unaffected in any case.

Err, yes. I forgot that altering the default of an existing column
doesn't set the default for existing values. Sorry for the noise.

Greetings,

Andres Freund