PostgreSQL: how to fetch previous insert timestamp

Question

Suppose that I have table

create table foo (
 insert_current timestamp default now(),
 insert_previous timestamp,
 bar int primary key,
 baz varchar(10)
);

The data will be replaced now and then when fresh batch arrived. I would like to keep track on when previous batch was inserted as well as current timestamp. What would be a good way to do this?

Answer 1

I would create a trigger that automatically sets insert_current and insert_previous on UPDATE statements.

First, create the trigger function:

CREATE OR REPLACE FUNCTION do_update() RETURNS "trigger"
AS $$
BEGIN
 NEW.insert_previous := OLD.insert_current;
 NEW.insert_current := NOW();
 return NEW;
END;
$$
LANGUAGE plpgsql;

Then add the trigger to your table:

CREATE TRIGGER do_update
BEFORE UPDATE ON foo
FOR EACH ROW
EXECUTE PROCEDURE do_update();

Answer 2

When you update a row, copy the value of insert_current to insert_previous and set insert_current to the current timestamp:

UPDATE foo
 SET baz = 'whatever',
 insert_previous = insert_current,
 insert_current = NOW()
 WHERE bar = 1;