Add timestamp column with default NOW() for new rows only

I have a table that has thousands of rows. Since the table wasn't constructed with created_at column initially, there is no way of getting their creation timestamp. It is crucial though to start getting the timestamps for future rows.

Is there a way I can add a timestamp column with default value NOW() so that it won't populate the values to previous rows but only for the future ones?

If I do the ALTER query, it populates all rows with timestamp:

ALTER TABLE mytable ADD COLUMN created_at TIMESTAMP DEFAULT NOW()

You need to add the column with a default of null, then alter the column to have default now().

ALTER TABLE mytable ADD COLUMN created_at TIMESTAMP;
ALTER TABLE mytable ALTER COLUMN created_at SET DEFAULT now();

You could add the default rule with the alter table,

ALTER TABLE mytable ADD COLUMN created_at TIMESTAMP DEFAULT NOW()

then immediately set to null all the current existing rows:

UPDATE mytable SET created_at = NULL

Then from this point on the DEFAULT will take effect.

For example, I will create a table called users as below and give a column named date a default value NOW()

create table users_parent (
    user_id     varchar(50),
    full_name   varchar(240),
    login_id_1  varchar(50),
    date        timestamp NOT NULL DEFAULT NOW()
);

Thanks

minor optimization.

select pg_typeof(now());   --returns: timestamp with time zone. So now include timezone.        

So better with timestamptz.

begin;
ALTER TABLE mytable ADD COLUMN created_at TIMESTAMPTZ;
ALTER TABLE mytable ALTER COLUMN created_at SET DEFAULT now();
commit;

Try something like:-

ALTER TABLE table_name ADD  CONSTRAINT [DF_table_name_Created] 
DEFAULT (getdate()) FOR [created_at];

replacing table_name with the name of your table.

This code worked for me

> ALTER TABLE ABC ADD COLUMN recievedon TIMESTAMP DEFAULT NOW()