SELECT U.SURNAME_WITH_INITIALS
  FROM SYS$CONFIG$DATASET D
    INNER JOIN DIR$SECURITY$DB_USER U ON U.ID = D.ID_CREATOR

CREATE TABLE test1
(
    user_id INTEGER NOT NULL,
    user_name VARCHAR(60) CHARACTER SET utf8,
  CONSTRAINT test1__pk PRIMARY KEY (user_id)
);
COMMIT WORK;

INSERT INTO test1 (user_id, user_name) VALUES (1, 'Иванов П.П.');
COMMIT WORK;


SET TERM ^;
CREATE OR ALTER PROCEDURE user__get
  (
    user_id INTEGER
  )
  RETURNS
  (
    user_name VARCHAR(60) CHARACTER SET utf8
  )
AS
BEGIN
  SELECT user_name
    FROM test1
    WHERE user_id = :user_id
    INTO: user_name;
  SUSPEND;
END^
SET TERM ;^
COMMIT WORK;

CREATE TABLE test2
(
    doc_id    INTEGER NOT NULL,
    doc_name  VARCHAR(60) CHARACTER SET utf8,
    user_id   INTEGER NOT NULL,
    user_name COMPUTED BY ((SELECT user_name FROM user__get(user_id))),
  CONSTRAINT test2__pk PRIMARY KEY (doc_id),
  CONSTRAINT test2__fk__test1 FOREIGN KEY (user_id)
    REFERENCES test1 (user_id) ON UPDATE CASCADE ON DELETE SET NULL
);
COMMIT WORK;

INSERT INTO test2 (doc_id, doc_name, user_id) VALUES (1, 'Документ №1', 1);
COMMIT WORK;