the behaviour of timestamp on postgres.

Dear my friends...

I created some tables with field timestamp (datatype
also timestamp). I mean, I want to have the data when
each record inserted or modified in the tables.

on MysQL, I just need to define the column (field)
with datatype "timestamp" and that's all. each time
new record inserted than the timestamp value will be
inserted automaticall. also for the data modification,
each time the data in the record modified than the
value of timestamp column will be modified
automatically.

How is the behaviour of the timestamp on postgres? I
have define the datatype of the column with
"timestamp" but each time I inserted a new record into
the table than the timestamp column (with datatype
"timestamp") stays empty.

How can I make the postgres complete the value of the
timestamp field automatically?

Please let me know.

Thank you very much in advance.

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