Storing old and new tuple values after an UPDATE, INSERT or DELETE

Is it possible to modify source code in executor to keep a log of old tuples
and new tuples after an UPDATE, INSERT or DELETE command is executed? (so
basically Store Old Tuple Values -> UPDATE/INSERT/DELETE Command -> Store
New Tuple Values)

For example is it possible to modify the execMain.c file in
backend/executor/ such that before the ExecutorRun you save the tuples in a
log file and after running ExecutorRun you log the new tuples also.
How can this be achieved  (if at all possible)?
 If it is not possible to do this by modifying execMain.c file how else can
I do it? (may be nodeModifyTable.c)

Please correct me if my understanding is wrong. I am new to Postgresql and
to source code programming specially.
I would really appreciate it a lot. Thank you in advance.
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