Question about no unchanging update rule + ALTER

Josh Trutwin wrote:

I found the following on a blog post
(http://www.depesz.com/index.php/2007/09/08/avoiding-empty-updates/)
which had a rule to prevent empty updates:

CREATE RULE no_unchanging_updates AS
ON UPDATE
TO test_table
WHERE ROW(OLD.*) IS NOT DISTINCT FROM ROW(NEW.*)
DO INSTEAD NOTHING;

Works great, but problem comes when I alter the table and add a new
column, it appears the rule doesn't allow an update after adding a
new column via ALTER TABLE ADD COLUMN.

I created the rule above, then did:

ALTER TABLE test_table ADD COLUMN foo TEXT;

=> UPDATE test_table SET foo = 'bar';
UPDATE 0

When doing a \d on the table I notice the rule is expanded at the
time of creation to include each column in an expression, but it is
not updated from the ALTER TABLE command.

Do I have to drop and recreate this rule after every ALTER TABLE
ADD/DELETE column?

Quite possibly - I seem to remember that id *does* expand the * to an
explicit list of columns. That's what you want sometimes. If the whole
point of the view is to provide a stable interface to an application,
you don't want it changing when you change underlying tables.

Or would the following trigger (also found on
blog post) be a better solution as my app is for a "plugin" builder
where adding/deleting/changing fields is common:

CREATE OR REPLACE FUNCTION prevent_empty_updates() RETURNS trigger as
$BODY$
DECLARE
BEGIN
IF ROW(OLD.*) IS DISTINCT FROM ROW(NEW.*) THEN
RETURN NEW;
END IF;
RETURN NULL;
END;
$BODY$ language plpgsql;

CREATE TRIGGER prevent_empty_updates BEFORE UPDATE ON test FOR EACH
ROW EXECUTE PROCEDURE prevent_empty_updates();

Actually after writing this, this TOO does not seem to work after an
ADD COLUMN. :/ Any suggestions?

Try disconnecting and reconnecting to the database - that should do it.
The function will be "compiled" the first time it is called in a session
, so the * is probably getting expanded then. There's been a lot of work
done to provide automatic re-planning in these sort of situations, but
maybe you're hitting a corner-case.

--
Richard Huxton
Archonet Ltd

On Fri, 27 Feb 2009 09:34:08 +0000
Richard Huxton <dev@archonet.com> wrote:

CREATE TRIGGER prevent_empty_updates BEFORE UPDATE ON test FOR
EACH ROW EXECUTE PROCEDURE prevent_empty_updates();

Actually after writing this, this TOO does not seem to work after
an ADD COLUMN. :/ Any suggestions?

Try disconnecting and reconnecting to the database - that should do
it. The function will be "compiled" the first time it is called in
a session , so the * is probably getting expanded then. There's
been a lot of work done to provide automatic re-planning in these
sort of situations, but maybe you're hitting a corner-case.

Thanks for the info - I'll test out the disconnect and report back.

Josh.