DROP CONSTRAINT, printing a notice and/or skipping when no action is taken

Hello. I noticed a small opportunity for a possible enhancement to DROP
DEFAULT, and wanted to share the idea. Apologies if this idea was suggested
before, I tried a basic search for pgsql-hackers similar things but didn’t
find a hit.

I noticed when running an ALTER TABLE with DROP DEFAULT, whether the column
default exists or not, ALTER TABLE is always printed as the result. This is
arguably slightly confusing, because it’s unclear if anything was done. In
the scenario where there is no column default, there isn’t a message saying
“skipped” or something equivalent, indicating that there was no default
that was dropped. Some of the commands in Postgres do have this kind of
feedback, so it seems like an opportunity for greater consistency.

For example: if I create a column default, or repeatedly run the following
ALTER TABLE statements for the "id_new" column, I always get ALTER TABLE
back.

ALTER TABLE my_table ALTER COLUMN id_new DROP DEFAULT;

ALTER TABLE

ALTER TABLE my_table ALTER COLUMN id_new DROP DEFAULT;

ALTER TABLE

ALTER TABLE my_table ALTER COLUMN id_new DROP DEFAULT;

ALTER TABLE

An opportunity would be to add a NOTICE type of message when ALTER TABLE
ALTER COLUMN DROP DEFAULT is issued, at least when no column default
exists, and no action was taken. In that scenario, the operation could
possibly be skipped altogether, which might have some additional benefits.

As a refreshed on a “Notice” type of message example, here’s one when
adding an index and using the "if not exists" clause (an equivalent "if not
exists" clause does not exist for DROP DEFAULT to my knowledge):

-- an index called “foo” already exists

psql> create index if not exists foo on organizations (id);

NOTICE: relation "foo" already exists, skipping

CREATE INDEX

The message being “NOTICE: relation "foo" already exists, skipping”

A similar message for DROP DEFAULT might look like:

“NOTICE: default does not exist, skipping”

Or an alternative that includes the column name might look like:

“NOTICE: default does not exist for column id_new, skipping”

Or another alternative might be a new (non-standard?) "if exists" clause
for DROP DEFAULT. Example:

ALTER TABLE my_table ALTER COLUMN id_new DROP DEFAULT IF EXISTS;

-- Or an alternative placement of the "if exists" clause, because I don’t
really know where it would go:

ALTER TABLE my_table ALTER COLUMN id_new DROP IF EXISTS DEFAULT;

Thanks!

- Andrew