CREATE TABLE TMP_DATE1
(
  DATE1 DATE,
  DATE2 DATE
);

EXECUTE BLOCK
AS
  DECLARE VARIABLE D DATE;
BEGIN
  D = '01.01.2008';
  WHILE (D < '01.08.2008') DO BEGIN
    INSERT INTO TMP_DATE1(DATE1, DATE2)
      VALUES(:D, :D + 100);
      D = D + 1;
  END
END;

CREATE INDEX TMP_DATE1_IDX1 ON TMP_DATE1 COMPUTED BY (DATE1+0)


---------------------------------------

without index

SELECT count(*) FROM TMP_DATE1 T WHERE '01.03.2008' BETWEEN T.DATE1 AND T.DATE2;

count = 61

---------------------------------------

with index

SELECT count(*) FROM TMP_DATE1 T WHERE '01.03.2008' BETWEEN T.DATE1+0 AND T.DATE2

count = 1