Re: [COMMITTERS] pgsql: Remove arbitrary ALTER TABLE .. ADD COLUMN restriction.

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> Considering the number of OTHER places we'd have to break backward
> compatibility, one more wouldn't bother me any, but apparently that's
> just me.

Well, again, it'd be all right with me if we were going to get any
meaningful increment in functionality out of it, but we aren't. If you
add the column and the default in separate steps, you get the same
result and all the behavior is spec-compliant.

There's some history here that you may not be familiar with --- we used
to have exactly this bug in regards to the mainline ALTER TABLE case.
Observe the results in PG 7.1:

regression=# create table foo(f1 int);
CREATE
regression=# insert into foo values(1);
INSERT 3151259 1
regression=# insert into foo values(2);
INSERT 3151260 1
regression=# alter table foo add column f2 int default 2;
ALTER
regression=# select * from foo;
f1 | f2
----+----
1 |
2 |
(2 rows)

In 7.2 through 7.4 the ALTER failed instead:
regression=# alter table foo add column f2 int default 2;
ERROR: Adding columns with defaults is not implemented.
Add the column, then use ALTER TABLE SET DEFAULT.

and by 8.0 we'd finally made it work per spec. But we got lots of flak
meanwhile from people who'd gotten used to the old behavior. So those
of us who were around then aren't eager to repeat that. The code you're
complaining about now was put in only a month after we got that problem
fixed, so the issue was plenty fresh in mind at the time.

regards, tom lane