Re: Rules: A Modest Proposal

>>>>> "Greg" == "Greg Sabino Mullane" <greg(at)turnstep(dot)com> writes:

>> They're mostly a foot-gun.

Greg> Lots of things in Postgres could be considered potential foot
Greg> guns. Frankly, I don't think rules are even near the top of
Greg> such a list. Can you give examples of rule foot guns?

There are so many it's hard to know where to start.

Here are a couple of the more common ones:

1) any reference in an insert rule to NEW.col where col has a volatile
default, or the expression in the insert statement was volatile, or
the expression's value is changed by the insert, will do the wrong
thing:

create table t (a integer);
create table t_log (a integer);
create rule t_ins AS ON insert TO t do also insert into t_log values (NEW.a);
insert into t values (floor(random()*1000)::integer);
select * from t;
a
----
33
(1 row)

select * from t_log;
a
-----
392
(1 row)

(think "nextval" or "uuid_generate_*" for more realistic examples)

2) any rule with multiple actions, each action is affected by the results of
the previous ones. A classic example of this is in the use of OLD in
delete or update rules; OLD _does not return a row_ if a previous action
in the rule deleted the row or updated it so that it no longer matches.

--
Andrew (irc:RhodiumToad)