Re: trigger for TRUNCATE?

tgl(at)sss(dot)pgh(dot)pa(dot)us (Tom Lane) writes:
> Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
>> Gerardo Herzig escribió:
>>> Yes, the TRUNCATE statement is not sql ansi, maybe is a more low level
>>> thing than i think.
>
>> TRUNCATE currently does not fire triggers, but that doesn't mean it's
>> impossible to do it. I think it would be fairly easy to add support
>> for that.
>
> The entire point of TRUNCATE is to not do a table scan, so making it
> fire per-row triggers seems pretty misguided to me.
>
> We could maybe make it fire per-statement ON DELETE triggers, but
> there's a future-proofing pitfall in that: someday it'd be nice
> for statement-level triggers to have access to the set of deleted rows,
> and then you'd be stuck either scanning the table or having TRUNCATE
> act differently from plain DELETE.
>
> My feeling is that if you want to know what was deleted, you shouldn't
> use TRUNCATE.

No, what would be nice to have is NOT per-row triggering, but rather
simply the ability to run a stored function ON TRUNCATE.

This would be useful for Slony-I:

- On replica nodes, we might add a trigger:
create trigger t_trunc before truncate on my_table for each statement execute _sl_cluster.deny_truncate();
which would raise the error: "Slony-I: Cannot TRUNCATE on subscriber node!"

- On the "master" we might add a trigger:
create trigger t_trunc before truncate on my_table for each statement execute _sl_cluster.createEvent('sl_cluster', 'TRUNCATE_TABLE', 14);
which would generate a 'TRUNCATE_TABLE' event that would tell other nodes to truncate table #14, that is, my_table.

For the case where people want to track "COUNT(*)" on a table using
triggers, TRUNCATE presently throws that off. With a truncate
trigger, we might implement the following:

create trigger t_trunc before truncate on my_table for each statement execute purge_table('public', 'my_table');

create or replace function purge_table (text,text) returns null as $$
delete from count_summary_table where nspname = $1 and tabname = $2
$$ language sql;

That's three use cases, so far, none of which expect to have access to
the data that is being truncated.
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