Postgres 13

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CREATE TABLE foo
(
    id varchar(10),
    foo_count int,
    actual_date date
);

insert into foo values
('234534', 100, '2017-01-01'), 
('234534', 200, '2017-01-02'); 

SELECT * FROM foo;

CREATE TABLE



INSERT 0 2




id
foo_count
actual_date


234534
100
2017-01-01

234534
200
2017-01-02



SELECT 2


CREATE FUNCTION insert_missing()
    RETURNS TRIGGER AS
$$
DECLARE
    max_record record;
BEGIN
    SELECT
        id,
        foo_count,
        actual_date
    FROM
        foo
    ORDER BY actual_date DESC
    LIMIT 1
    INTO max_record;

    IF (NEW.actual_date - 1 > max_record.actual_date) THEN
        INSERT INTO foo
        SELECT 
            max_record.id,
            max_record.foo_count,
            generate_series(max_record.actual_date + 1, NEW.actual_date - 1, interval '1 day');
    END IF;

    RETURN NEW;
END;
$$ language 'plpgsql';

CREATE FUNCTION


CREATE TRIGGER insert_missing
  BEFORE INSERT
  ON foo
  FOR EACH ROW
  EXECUTE PROCEDURE insert_missing();

CREATE TRIGGER


insert into foo values
('234534', 300, '2017-01-03');

SELECT * FROM foo;

INSERT 0 1




id
foo_count
actual_date


234534
100
2017-01-01

234534
200
2017-01-02

234534
300
2017-01-03



SELECT 3


insert into foo values
('234534', 180, '2017-01-08');

SELECT * FROM foo

INSERT 0 1




id
foo_count
actual_date


234534
100
2017-01-01

234534
200
2017-01-02

234534
300
2017-01-03

234534
300
2017-01-04

234534
300
2017-01-05

234534
300
2017-01-06

234534
300
2017-01-07

234534
180
2017-01-08



SELECT 8

id	foo_count	actual_date
234534	100	2017-01-01
234534	200	2017-01-02
234534	300	2017-01-03
234534	300	2017-01-04
234534	300	2017-01-05
234534	300	2017-01-06
234534	300	2017-01-07
234534	180	2017-01-08