PERCENTILE_DISC() now return an error on temporal and string input:

CREATE OR REPLACE TABLE t1 (name CHAR(30), star_rating TIME);

INSERT INTO t1 VALUES ('Lord of the Ladybirds', '00:00:05');

INSERT INTO t1 VALUES ('Lord of the Ladybirds', '00:00:03');

INSERT INTO t1 VALUES ('Lady of the Flies', '00:00:01');

INSERT INTO t1 VALUES ('Lady of the Flies', '00:00:02');

INSERT INTO t1 VALUES ('Lady of the Flies', '00:00:05');

SELECT name, PERCENTILE_DISC(0.5)

  WITHIN GROUP (ORDER BY star_rating)

  OVER (PARTITION BY name) AS pc FROM t1;

ERROR 4101 (HY000): Numeric datatype is required for percentile_disc function

It should be extended to support all string and temporal data types.

Note, PERCENTILE_DISC() previously crashed on temporal input. The patch for MDEV-20272 fixed the crash and made it return an error instead. Allowing temporal and string types in PERCENTILE_DISC() needs some refactoring, which was not desirable in 10.3.

Refactoring needed:
Item_sum_percentile_disc currently recursively derives from Item_sum_num, which cannot return string/temporal values by design.
In order to support string/temporal data types in PERCENTILE_DISC(), Item_sum_percentile_disc should be moved to the Item_sum_hybrid
branch in the hierarchy.

It's not desirable to do such refactoring in 10.3 or 10.4.