Re: Trigger with WHEN clause (WIP)

On Fri, 2009-10-16 at 10:14 +0900, Itagaki Takahiro wrote:
> Tom Lane <tgl@sss.pgh.pa.us> wrote:
> 
> > Itagaki Takahiro <itagaki.takahiro@oss.ntt.co.jp> writes:
> > > I think there is a benefit to provide WHEN cluase at least
> > > for compatibility with other DBMSs, even through we can move
> > > the expressions into the body of trigger functions.
> > 
> > This seems to me to be a lot of code to accomplish nothing useful.
> > It will always be the case that any nontrivial logic has to be done
> > inside the trigger.  And the compatibility argument is entirely
> > pointless given the lack of compatibility in the trigger function
> > itself.
> 
> I see that WHEN cluase is not always needed,
> but I think there are several benefits to have it:
> 
>   * WHEN cluase is in SQL standard.
> 
>   * We could recheck trigger conditions when NEW tuple is modified.
>     (not yet implemented in the patch, though)
>         http://archives.postgresql.org/pgsql-hackers/2009-09/msg00286.php
> 
>   * As for compatibility, it is easy for typical users to move trigger
>     bodies into a function, but they don't like to merge the condition
>     and the bodies into a function in my experience.

+1 because

* It is SQL Standard
* Other RDBMS support it (Oracle, DB2)
* It will reduce size of in-memory pending trigger list (for large
statements) and avoid invoking run-time of function handlers, important
for heavier PLs (for small statements)

I also support addition of INSTEAD OF triggers for first two reasons and
because it may be an easier route to allow updateable views.

-- Simon Riggs           www.2ndQuadrant.com