CREATE TABLE audit_log (
    id SERIAL PRIMARY KEY,
    schema_name TEXT NOT NULL,
    table_name TEXT NOT NULL,
    operation CHAR(1) NOT NULL,
    new_data JSONB,
    old_data JSONB,
    changed_at TIMESTAMP NOT NULL,
    changed_by TEXT
);

-- create function definition
CREATE OR REPLACE FUNCTION audit_log_function() RETURNS TRIGGER AS $$
BEGIN
    IF (TG_OP = 'UPDATE') THEN
        INSERT INTO audit_log (schema_name, table_name, operation, new_data, old_data, changed_at, changed_by)
        VALUES (TG_TABLE_SCHEMA, TG_TABLE_NAME, 'U', to_jsonb(NEW), to_jsonb(OLD), current_timestamp, current_user);
        RETURN NEW;
    ELSIF (TG_OP = 'INSERT') THEN
        INSERT INTO audit_log (schema_name, table_name, operation, new_data, changed_at, changed_by)
        VALUES (TG_TABLE_SCHEMA, TG_TABLE_NAME, 'I', to_jsonb(NEW), current_timestamp, current_user);
        RETURN NEW;
    ELSE
        RAISE EXCEPTION 'This trigger should only be used for UPDATE and INSERT operations.';
    END IF;
END;
$$ LANGUAGE plpgsql;

-- create trigger for all tables
DO $$
DECLARE
   table_name TEXT;
BEGIN
   FOR table_name IN (SELECT t.table_name FROM information_schema.tables t WHERE t.table_schema = 'public' AND t.table_type = 'BASE TABLE' AND t.table_name != 'audit_log')
   LOOP
      EXECUTE format('CREATE TRIGGER audit_log_trigger
                      AFTER INSERT OR UPDATE ON %I
                      FOR EACH ROW
                      EXECUTE FUNCTION audit_log_function();', table_name);
   END LOOP;
END $$;