We are building similar functionality for our own postgres setup right now.

Of course, postgres is very powerful and you can implement anything like this in many different ways. But at the same time, maintaining DDL history and tracking major changes to the database is a very common requirement, and unfortunately many people don't realize that until they learned that lesson the hard way.

Relatedly are not DDL changes per-se, but big/important db operations that you want to also keep a record of so that you can look back and understand why something changed. I am not sure if this is the right term, but basically when we update our pricing model or skus, or set custom pricing for someone, we want those updates to be "auditable".

Actually, I think this is a relatively common use case too: a fully relational model often leaves you with a large number of "static" tables that only change when you're making updates to your application. They support the smaller number of big, important, dynamic tables that your application is regularly modifying. If you had the foresight to recognize that you'd likely need to change those static tables in the future, you probably organized them so you could do so by adding new rows. It is not quite a DDL change but it is a big, risky change to your application logic that only happens rarely, and you basically just want to keep a list of all those big changes in case things get messed up or you find yourself unable to make sense of older data.

There's lots of extensions that automatically create audit tables and record history in them. The trick is to a) create audit tables for all _existing_ tables, b) create an event trigger so you can have audit tables created automatically for any future new tables. Here's an example of how to do it: https://github.com/twosigma/postgresql-contrib/blob/master/a...

Another thing you might do is to go with a schema that follows the event shipping pattern. In this pattern you have the "truth" held in insert-only tables (deletes and updates not allowed), then turn those "event" tables into ones you can query naturally using VIEWs, MATERIALIZED VIEWs, or live tables that you update with triggers on the event tables. Then your event tables _are your history/audit_ tables.

    CREATE TABLE public.audit (
        id uuid NOT NULL,
        created_time timestamp without time zone DEFAULT now() NOT NULL,
        schema_name text NOT NULL,
        table_name text NOT NULL,
        record_id uuid NOT NULL,
        user_name text,
        action text NOT NULL,
        old_data jsonb,
        new_data jsonb
    );

    CREATE OR REPLACE FUNCTION audit_if_modified_func() RETURNS TRIGGER AS $body$
    DECLARE
        v_old_data JSONB;
        v_new_data JSONB;
    BEGIN
        IF (TG_OP = 'UPDATE') THEN
            v_old_data := to_jsonb(OLD.*);
            v_new_data := to_jsonb(NEW.*);
            IF (TG_TABLE_NAME::TEXT = 'users') THEN
                v_old_data = v_old_data - 'last_login_time';
                v_new_data = v_new_data - 'last_login_time';
            END IF;
            IF (v_old_data <> v_new_data) THEN
                INSERT INTO audit (id,record_id,schema_name,table_name,user_name,action,old_data,new_data)
                    VALUES (uuid_generate_v4(), NEW.id, TG_TABLE_SCHEMA::TEXT, TG_TABLE_NAME::TEXT,
                            session_user::TEXT, substring(TG_OP,1,1), v_old_data, v_new_data);
            END IF;
            RETURN NEW;
        ELSIF (TG_OP = 'DELETE') THEN
            v_old_data := to_jsonb(OLD.*);
            INSERT INTO audit (id, record_id,schema_name,table_name,user_name,action,old_data)
            VALUES (uuid_generate_v4(), OLD.id, TG_TABLE_SCHEMA::TEXT,TG_TABLE_NAME::TEXT,session_user::TEXT,substring(TG_OP,1,1),v_old_data);
            RETURN OLD;
        ELSIF (TG_OP = 'INSERT') THEN
            v_new_data := to_jsonb(NEW.*);
            INSERT INTO audit (id, record_id,schema_name,table_name,user_name,action,new_data)
            VALUES (uuid_generate_v4(), NEW.id, TG_TABLE_SCHEMA::TEXT,TG_TABLE_NAME::TEXT,session_user::TEXT,substring(TG_OP,1,1),v_new_data);
            RETURN NEW;
        ELSE
            RAISE WARNING '[AUDIT_IF_MODIFIED_FUNC] - Other action occurred: % at %',TG_OP,now();
            RETURN NULL;
        END IF;
    
    EXCEPTION
        WHEN data_exception THEN
            RAISE WARNING '[AUDIT_IF_MODIFIED_FUNC] - UDF ERROR [DATA EXCEPTION] - SQLSTATE: % SQLERRM: %',SQLSTATE,SQLERRM;
            RETURN NULL;
        WHEN unique_violation THEN
            RAISE WARNING '[AUDIT_IF_MODIFIED_FUNC] - UDF ERROR [UNIQUE] - SQLSTATE: % SQLERRM: %',SQLSTATE,SQLERRM;
            RETURN NULL;
        WHEN OTHERS THEN
            RAISE WARNING '[AUDIT_IF_MODIFIED_FUNC] - UDF ERROR [OTHER] - SQLSTATE: % SQLERRM: %',SQLSTATE,SQLERRM;
            RETURN NULL;
    END;
    $body$
    LANGUAGE plpgsql
    SECURITY DEFINER;
    
    CREATE TRIGGER audit_logger_accounts
        AFTER INSERT OR UPDATE OR DELETE ON accounts
        FOR EACH ROW EXECUTE PROCEDURE audit_if_modified_func();

    CREATE TRIGGER audit_logger_users
        AFTER INSERT OR UPDATE OR DELETE ON users
        FOR EACH ROW EXECUTE PROCEDURE audit_if_modified_func();