SET TERM ^;
CREATE OR ALTER PROCEDURE getKey
  RETURNS
  (
    Id      type of column pool.id,
    Key     type of column pool.key,
    TmStmp  TIMESTAMP
  )
AS
BEGIN
  TmStmp = CURRENT_TIMESTAMP(3);
  UPDATE pool
    SET tmstmp = NULL
    WHERE dt_use IS NULL
      AND :TmStmp - tmstmp > 0.00138;
  FOR
      SELECT ROWS 1 id, key
        FROM pool
        WHERE dt_use IS NULL
          AND tmstmp IS NULL
        ORDER BY key
        FOR UPDATE WITH LOCK
        INTO: Id, Key
    DO
      BEGIN
        UPDATE pool
          SET tmstmp = :TmStmp
          WHERE id = :Id;
        SUSPEND;
      END
END^
SET TERM ;^

UPDATE pool
  SET dt_use = 'Y'
  WHERE id = :Id
    AND tmstmp IS NOT NULL AND tmstmp = :TmStmp;

EXECUTE BLOCK
AS
  DECLARE VARIABLE Id TYPE OF COLUMN pool.id DEFAULT NULL;
  DECLARE VARIABLE Key TYPE OF COLUMN pool.key DEFAULT NULL;
  DECLARE VARIABLE TmStmp TIMESTAMP DEFAULT NULL;
BEGIN
  IN AUTONOMOUS TRANSACTION DO
    SELECT Id, Key, TmStmp
      FROM getKey
      INTO: Id, Key, TmStmp;
  .
  . -- do something
  .
  IF (:Id IS NOT NULL) THEN
    UPDATE pool
      SET dt_use = 'Y'
      WHERE id = :Id
        AND tmstmp IS NOT NULL AND tmstmp = :TmStmp;
END

AND tmstmp IS NOT NULL AND tmstmp = :TmStmp;