Re: Trigger with WHEN clause (WIP)

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Itagaki Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)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.

* As for performance, I don't have any evidence. But there was a
discussion about benefits of writing partitioning trigger function
with C instead of PL/pgSQL. He said pgplsql is slow.
http://archives.postgresql.org/pgsql-hackers/2008-12/msg01221.php
Also, to separate trigger bodies and conditions are useful when we
write triggers in C because we don't have to recomplie the code.

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center