Postgresql Current timestamp on Update

Question

What is the postgres equivalent of the below mysql code

CREATE TABLE t1 (
  created TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  modified TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);
CREATE TABLE t2 (
  created TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  modified TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);

As per Alex Brasetvik answer below, it seems i should go with triggers, my problem is i have a number of tables t1, t2... with created and modified fields, is it possible to write a generalized procedure?

--update Almost ready

CREATE FUNCTION update_timestamp() RETURNS trigger AS $update_timestamp$
    BEGIN
        NEW.modified := current_timestamp;
        RETURN NEW;
    END;
$update_timestamp$ LANGUAGE plpgsql;

CREATE TRIGGER update_timestamp BEFORE INSERT OR UPDATE ON t1
    FOR EACH ROW EXECUTE PROCEDURE update_timestamp();
CREATE TRIGGER update_timestamp BEFORE INSERT OR UPDATE ON t2
    FOR EACH ROW EXECUTE PROCEDURE update_timestamp();

Answer 1

Just make sure all tables have the same columnname:

CREATE OR REPLACE FUNCTION upd_timestamp() RETURNS TRIGGER 
LANGUAGE plpgsql
AS
$$
BEGIN
    NEW.modified = CURRENT_TIMESTAMP;
    RETURN NEW;
END;
$$;

CREATE TRIGGER t_name
  BEFORE UPDATE
  ON tablename
  FOR EACH ROW
  EXECUTE PROCEDURE upd_timestamp();

Answer 2

Thank you for the information Mithun and Alex Brasetvik.

I'd like to add one minor tweak to the trigger. Since we mostly likely want the modified column to store the timestamp when the row was last changed, not when it was the target of an UPDATE statement, we have to compare the new and the old value of the row. We update the modified column only if these two values differ.

CREATE OR REPLACE FUNCTION update_modified_timestamp() RETURNS TRIGGER 
LANGUAGE plpgsql
AS
$$
BEGIN
    IF (NEW != OLD) THEN
        NEW.modified = CURRENT_TIMESTAMP;
        RETURN NEW;
    END IF;
    RETURN OLD;
END;
$$;

This trigger ensures that the modified column is updated only if the UPDATE operation actually changes the values stored in the row.

Answer 3

Update it with a trigger. Documentation and examples.

Answer 4

To further improve, the answer given by @Lauri Silvennoinen:

This trigger uses the WHEN clause as recommended by the official docs to check for changes in the row even before calling the specified function.

CREATE OR REPLACE FUNCTION update_modified_timestamp() RETURNS TRIGGER 
LANGUAGE plpgsql
AS
$$
BEGIN
    NEW.modified = CURRENT_TIMESTAMP;
    RETURN NEW;
END;
$$;

CREATE OR REPLACE TRIGGER tg_update_modified
    AFTER UPDATE ON table_name
    FOR EACH ROW
    WHEN (OLD.* IS DISTINCT FROM NEW.*)
    EXECUTE FUNCTION update_modified_timestamp();