Re: Trigger with WHEN clause (WIP)

On Fri, 2009-10-16 at 10:02 -0400, Tom Lane wrote:
> Peter Eisentraut <peter_e(at)gmx(dot)net> writes:
> > On Fri, 2009-10-16 at 09:31 +0100, Simon Riggs wrote:
> >> * It will reduce size of in-memory pending trigger list (for large
> >> statements)
>
> > But this won't be the outcome when it's implemented the way it is being
> > proposed, which checks the where clause just before executing the
> > trigger function.
>
> Hm, the feature could actually be worth something if it allows
> conditions to be checked before an AFTER trigger event gets pushed
> into the event list. However, if it's not doing that ...
>
> I note BTW that we have some ad-hoc logic already that arranges to
> suppress queuing of AFTER events for FK triggers, if the FK column
> value has not changed. It might be interesting to look at whether
> that hack could be unified with the user-accessible feature. It's
> essentially a WHEN OLD.x IS NOT DISTINCT FROM NEW.x test.

If the function is idempotent then we can also optimise away multiple
calls by checking whether a similar call is already queued.

--
Simon Riggs www.2ndQuadrant.com