Consider the return value of deterministic functions to be invariant if all its arguments are invariant.

[sim1984]

Currently, the return result of deterministic functions is considered invariant only if the function contains no arguments. The implementation with a results cache storing <argument_values, result_value> pairs is quite complex. However, we can consider a simpler option: if the arguments of a deterministic function are invariants, such as literals or input parameters, then the return value of that function is also an invariant.

Simple example:

CREATE EXCEPTION E_INVALID_RACETIME_FORMAT '~Incorrect friskiness format~';

CREATE TABLE BIG_TABLE (
  ID BIGINT NOT NULL, 
  TIME_PASSED NUMERIC(18, 3),
  CONSTRAINT PK_BIG_TABLE PRIMARY KEY(ID)
);

SET TERM ^;

CREATE OR ALTER FUNCTION SF_RACETIME_TO_SEC (
    RACETIME VARCHAR(9))
RETURNS NUMERIC(18,3)
DETERMINISTIC
AS
DECLARE VARIABLE S VARCHAR(9);
DECLARE VARIABLE XCOMMAPOS SMALLINT;
DECLARE VARIABLE XDOTPOS SMALLINT;
DECLARE VARIABLE XMINUTE SMALLINT;
DECLARE VARIABLE XSECOND SMALLINT;
DECLARE VARIABLE XMS VARCHAR(9);
DECLARE VARIABLE XMILLISECOND SMALLINT;
DECLARE VARIABLE RTIME NUMERIC(18, 3);
BEGIN
  S = TRIM(RACETIME);
  S = TRIM('_' FROM S);
  RTIME = NULL;

  IF (TRIM(REPLACE(REPLACE(S, '.', ''), ',', '')) = '') THEN
    S = NULL;
  IF (S IS NOT NULL) THEN
  BEGIN
    IF (S NOT SIMILAR TO '[0-5]?[0-9].[0-5][0-9],[0-9]{1,3}') THEN
      EXCEPTION E_INVALID_RACETIME_FORMAT  '~Incorrect time format "' || RACETIME || '"~';

    XDOTPOS = POSITION('.', S);
    XCOMMAPOS = POSITION(',', S);
    XMINUTE = CAST(SUBSTRING(S FROM 1 FOR XDOTPOS) AS SMALLINT);

    XSECOND = CAST(SUBSTRING(S FROM XDOTPOS + 1 FOR XCOMMAPOS - XDOTPOS - 1) AS SMALLINT);

    XMS = TRIM(SUBSTRING(S FROM XCOMMAPOS + 1));
    XMILLISECOND = CAST(RPAD(XMS, 3, '0') AS SMALLINT);

    RTIME = 60 * XMINUTE + XSECOND + 0.001 * XMILLISECOND;
  END
  RETURN RTIME;
END^

EXECUTE BLOCK
AS
DECLARE ID BIGINT = 1;
BEGIN
  WHILE (ID <= 1000000) DO
  BEGIN
    INSERT INTO BIG_TABLE (ID, TIME_PASSED)
    VALUES (:ID, ROUND(150 - 50 * RAND(), 3));

    ID = ID + 1;
  END
END^

SET TERM ;^

COMMIT;

SELECT
COUNT(*)
FROM BIG_TABLE
WHERE BIG_TABLE.TIME_PASSED = SF_RACETIME_TO_SEC('02.01,6');

                COUNT
=====================
                   31

Current memory = 561798064
Delta memory = 358192
Max memory = 561813248
Elapsed time = 38.442 sec
Buffers = 32768
Reads = 0
Writes = 0
Fetches = 1015835

SQL> SELECT SF_RACETIME_TO_SEC('02.01,6') FROM RDB$DATABASE;

   SF_RACETIME_TO_SEC
=====================
              121.600

Current memory = 561794608
Delta memory = 10880
Max memory = 561884032
Elapsed time = 0.006 sec
Buffers = 32768
Reads = 0
Writes = 0
Fetches = 21

SELECT
COUNT(*)
FROM BIG_TABLE
WHERE BIG_TABLE.TIME_PASSED = 121.600;

                COUNT
=====================
                   31

Current memory = 561810448
Delta memory = 2784
Max memory = 561884032
Elapsed time = 0.393 sec
Buffers = 32768
Reads = 0
Writes = 0
Fetches = 1015569

If there is no index on the TIME_PASSED column, then SF_RACETIME_TO_SEC will be called for each record in the BIG_TABLE table.