SET TERM ^;
CREATE OR ALTER TRIGGER myTable__TR_BI
  FOR myTable
  ACTIVE
  BEFOR INSERT
  POSITION 0
AS
  DECLARE VARIABLE seqValue BIGINT DEFAULT NULL;
BEGIN
  .
  .
  .
  seqValue = GEN_LOCK(mySequence);    -- висим внутри функции, пока не заблокируем
                                      -- или пока наша транзакция не навернется по таймауту
  IF (NEW.Id IS NULL OR NEW.Id > :seqValue) THEN
    NEW.Id = GEN_ID(mySequence, 1);
  GEN_UNLOCK(mySequence);             -- освобождаем последовательность
  seqValue = NULL;
  .
  .
  .
  WHEN ANY DO
    BEGIN
      IF (:seqValue IS NOT NULL) THEN
        GEN_UNLOCK(mySequence);   -- Если 
      EXCEPTION;
    END
END^
SET TERM ;^

SET TERM ^;
CREATE OR ALTER TRIGGER myTable__TR_AD
  FOR myTable
  ACTIVE
  AFTER DELETE
  POSITION 32767
AS
  DECLARE VARIABLE seqValue BIGINT DEFAULT NULL;
BEGIN
  seqValue = GEN_LOCK(mySequence);    -- висим внутри функции, пока не заблокируем
                                      -- или пока наша транзакция не навернется по таймауту
  IF (NOT EXISTS(SELECT FIRST 1 * FROM myTable)) THEN
    seqValue = GEN_ID(mySequence, seqValue * -1)
  GEN_UNLOCK(mySequence);             -- освобождаем последовательность
  seqValue = NULL;
  WHEN ANY DO
    BEGIN
      IF (:seqValue IS NOT NULL) THEN
        GEN_UNLOCK(mySequence);
      EXCEPTION;
    END
END^
SET TERM ;^

SET TERM ^;
CREATE OR ALTER PROCEDURE shrinkMyTableId
AS
  DECLARE VARIABLE Id BIGINT DEFAULT NULL;
  DECLARE VARIABLE newId BIGINT DEFAULT 0;
  DECLARE VARIABLE seqValue BIGINT DEFAULT NULL;
BEGIN
  seqValue = GEN_LOCK(mySequence);   -- Блокируем изменение последовательности
  FOR
      SELECT Id, :newId + 1
        FROM myTable
        WHERE Id > 0
        ORDER BY Id ASC
        FOR UPDATE WITH LOCK
        INTO: Id, newId
    DO
      IF (:Id != :newId) THEN
        UPDATE myTable
          SET Id = :newId
          WHERE Id = :Id;
  seqValue = GEN_ID(mySequence, newId - seqValue);
  GEN_UNLOCK(mySequence);
  seqValue = NULL;
  WHEN ANY DO
    BEGIN
      IF (:seqValue IS NOT NULL) THEN
        GEN_UNLOCK(mySequence);
      EXCEPTION;
    END
END^
SET TERM ;^