CREATE DOMAIN BOOL AS CHAR(1) CHARACTER SET ASCII
  DEFAULT 'N' CHECK (VALUE = 'N' OR VALUE = 'Y');

CREATE TABLE "my_table"
(
    "id"      ID NOT NULL,
    "flag_1"  BOOL NOT NULL,
    "flag_2"  BOOL NOT NULL,
  CONSTRAINT "my_table__pk" PRIMARY KEY ("id")
);

CREATE DOMAIN UINT8 AS SMALLINT CHECK(VALUE BETWEEN 0 AND 255);

SET TERM ^;
CREATE FUNCTION test
  (
    par SMALLINT DEFAULT NULL
  )
  RETURNS BOOLEAN
AS
  DECLARE VARIABLE tmp UINT8;
BEGIN
  IF (:par IS NULL) THEN RETURN NULL;
  tmp = par;
  RETURN TRUE;
END^
SET TERM ;^

SELECT test(256) FROM oneRow;

The insert failed because a column definition includes validation constraints.
validation error for variable TMP, value "256".
At function 'TEST', line: 9, col: 3.

CREATE DOMAIN UINT8 AS SMALLINT CHECK(VALUE BETWEEN 0 AND 255);

CREATE TABLE test
(
    "id"      ID NOT NULL,

    /* следующее объявление работает */
    "field_1" COMPUTED BY (Cast(Bin_AND(Bin_SHR("id", 24), 0x0ff) AS UINT8)),

    /* следующее объявление НЕ работает */
--  "field_2" SMALLINT COMPUTED BY (Bin_AND(Bin_SHR("id", 24), 0x0ff))
--    CHECK("field_2" BETWEEN 0 AND 255),

    /* следующее объявление тоже НЕ работает */
--  "field_3" UINT8 COMPUTED BY (Bin_AND(Bin_SHR("id", 24), 0x0ff)),

    /* следующее объявление работает */
    "field_4" SMALLINT COMPUTED BY (Bin_AND(Bin_SHR("id", 24), 0x0ff))
  CONSTRAINT test__chk CHECK("field_4" BETWEEN 0 AND 255)
);

The insert failed because a column definition includes validation constraints.
validation error for variable TMP, value "256".
At function 'TEST', line: 9, col: 3.