How can I write trigger on a columns insert/update?
CREATE TRIGGER mytrigger
BEFORE
INSERT OR UPDATE OF mycolumn ON mytable
You don't I'm afraid. It's not a feature that's supported yet. You have to check which column(s) were changed in your functions.
It is on the TODO list though: http://www.postgresql.org/docs/faqs.TODO.html
-- Richard Huxton Archonet Ltd
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