Thread: Re: [COMMITTERS] pgsql: Add a WHEN clause to CREATE TRIGGER, allowing a boolean

On Fri, Nov 20, 2009 at 3:38 PM, Tom Lane <tgl@postgresql.org> wrote:
> Log Message:
> -----------
> Add a WHEN clause to CREATE TRIGGER, allowing a boolean expression to be
> checked to determine whether the trigger should be fired.
>
> For BEFORE triggers this is mostly a matter of spec compliance; but for AFTER
> triggers it can provide a noticeable performance improvement, since queuing of
> a deferred trigger event and re-fetching of the row(s) at end of statement can
> be short-circuited if the trigger does not need to be fired.
>
> Takahiro Itagaki, reviewed by KaiGai Kohei.

Random thought: would it be possible to use something like this to
optimize foreign key constraints, by not firing them if none of the
relevant columns have been updated?

...Robert

Robert Haas <robertmhaas@gmail.com> writes:
> Random thought: would it be possible to use something like this to
> optimize foreign key constraints, by not firing them if none of the
> relevant columns have been updated?

There already is code in there to do that; see RI_FKey_keyequal_upd_fk
and RI_FKey_keyequal_upd_pk.

Earlier in the discussion of this patch I had suggested eliminating the
special-case code for FKs in favor of using a WHEN clause, but that
would probably not fly unless we can make the WHEN-based implementation
just as fast as the bespoke code; which seems unlikely.
        regards, tom lane

On Fri, Nov 20, 2009 at 5:05 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> Random thought: would it be possible to use something like this to
>> optimize foreign key constraints, by not firing them if none of the
>> relevant columns have been updated?
>
> There already is code in there to do that; see RI_FKey_keyequal_upd_fk
> and RI_FKey_keyequal_upd_pk.

*scratches head*  Hmm, I see EXPLAIN showing time & calls logged
against fk triggers even when I don't update any columns, unless the
existing value is NULL.  But maybe I'm doing something strange, or
misinterpreting the output.

> Earlier in the discussion of this patch I had suggested eliminating the
> special-case code for FKs in favor of using a WHEN clause, but that
> would probably not fly unless we can make the WHEN-based implementation
> just as fast as the bespoke code; which seems unlikely.

Yeah.

...Robert