absolute or relative updates - Mailing list pgsql-general

I want to write a trigger that logs any changes that are made to a row.
I have the old an new row values, but I am more interested in knowing if
the update was absolute "UPDATE table SET col = 3" or relative "UPDATE
table SET col = col + 2".
Is the a way (any way) to access this type of information ? In any
language available (C, PLPGSQL, etc.)

Thanks
Razvan Radu