http://oncodesign.io/2016/09/23/better-zero-downtime-deployment-with-a-database-using-triggers-and-flyway/

V1__init.sql

CREATE TABLE PERSON (
  id bigserial primary key,
  first_name varchar(255) not null,
  last_name varchar(255) not null
);

V2__Add_surname(1).sql

ALTER TABLE PERSON ADD surname varchar(255);
UPDATE PERSON SET surname = last_name;
ALTER TABLE PERSON ALTER COLUMN surname set not null;

...

V2__Add_surname(2).sql

...

CREATE OR REPLACE FUNCTION insert_update_person()
RETURNS trigger AS
$$
BEGIN
    IF (TG_OP = 'INSERT') THEN
        -- if the record to be inserted contains last_name then that means app v1 is inserting so the value 
        -- must be copied to surname in order for app v2 to see the value
        IF NEW.last_name IS NOT NULL THEN
            NEW.surname = NEW.last_name;
        -- if the record to be inserted contains surname then that means app v2 is inserting so the value
	-- must be copied to last_name in order for app v1 to see the value
        ELSIF NEW.surname IS NOT NULL THEN
            NEW.last_name = NEW.surname;
        END IF;
    ELSE
        -- if the record to be updated contains last_name that is different from the updating record then
	-- that means app v1 is updating the record so copy the updated last_name value to surname allowing app v2 
	-- to see the updated value
        IF OLD.last_name IS DISTINCT FROM NEW.last_name THEN
            NEW.surname = NEW.last_name;
        -- if the record to be updated contains surname that is different from the updating record then
	-- that means app v2 is updating the record so copy the updated surname value to last_name allowing app v1
	-- to see the updated value           
        ELSIF OLD.surname IS DISTINCT FROM NEW.surname THEN
            NEW.last_name = NEW.surname;
        END IF;
    END IF;

    RETURN NEW;
END
$$ LANGUAGE plpgsql;

CREATE TRIGGER insert_update_person
    BEFORE INSERT OR UPDATE ON PERSON
    FOR EACH ROW
    EXECUTE PROCEDURE insert_update_person();