Here is a patch to provide WHEN clause on triggers discussed here:
http://archives.postgresql.org/pgsql-hackers/2009-10/msg00869.php
Of course we can check conditions in the body of triggers, but
there are some benefits to have separate WHEN clause for SQL standard
compliance, portability, and saving AFTER trigger queues.
There might be still incomplete and debatable part in the patch:
* Usages of TupleTableSlot and RangeTblEntry might be ugly.
Please notice me if there are better ways.
* There are no "retry" feature to evaluate condition when
the NEW tuple is modifed by another trigger. Triggers are
still executed in alphabetical order, but it is adjustable.
Comments welcome.
Regards,
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ITAGAKI Takahiro
NTT Open Source Software Center