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