Re: ALTER TABLE DDL Triggers?

richard.broersma@gmail.com ("Richard Broersma") writes:
> I don't believe that DDL Triggers exist, correct?

That is correct.[1]

The usual point is that you cannot attach triggers to pg_catalog
tables, which would be the "obvious" way of trying to notice DDL
changes.  (e.g. - by having triggers that would fire when pg_class or
pg_attribute are changed)

It would presumably not be _completely_ implausible to run a trigger
when a table was ALTERed; the trouble would be in evaluating the
semantics what OLD.* and NEW.* ought to contain, _if anything_.

If you took the [1] TRUNCATE approach, there isn't _anything_ (in
terms of OLD.*/NEW.* values) passed to the function; in that case, it
is sufficient to have something (like a function parameter) to
identify the table, and thus pass that info downstream (e.g. - with
replication, passing the TRUNCATE on to downstream nodes).  TRUNCATE
is pretty simple; we know well that all it's doing is to get rid of
all the contents of the table at a point in time.

In contrast, the effect of ALTER TABLE is to make near-arbitrary
alterations to pg_class, pg_attribute, and such, and there is, on the
one hand, no obvious semantic of what data to even imagine passing on,
and, on the other, a grand problem of reconstructing the change if you
*did* have access to those underlying tables.  That's usually where
the discussion peters out when people propose DDL triggers.

[1] Or about 99% so.  There is a change committed for 8.4 where
TRUNCATE can fire a trigger.  But it's somewhat disputable whether
TRUNCATE should properly be considered DDL or not.
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