Full support of operator IS NOT DISTINCT FROM <=>

[vdimir]

We recently added support of <=> operator for null-safe comparison on JOIN ON section. However, it's not implemented in SELECT list.
To do that we need to implement https://github.com/ClickHouse/ClickHouse/blob/61f0683f1cfe21566f363703221fa17e80902b87/src/Functions/isNotDistinctFrom.h function.

There're several options:

• Implement support inside https://github.com/ClickHouse/ClickHouse/blob/61f0683f1cfe21566f363703221fa17e80902b87/src/Functions/FunctionsComparison.h . Pass some flag to the class implementing equals to support null-safe version as well. This option is preferred unless too many changes inside FunctionsComparison are required.

• Or implement it separately in isNotDistinctFrom.cpp. We may reuse the existing equals function and handle the result null mask additionally. Overload for tuples may be required because both(NULL, 1) = (NULL, NULL) and (NULL, 1) = (NULL, 1) returns NULL, but `<=> should handle them differently checking values inside NULL. See https://github.com/ClickHouse/ClickHouse/blob/61f0683f1cfe21566f363703221fa17e80902b87/src/Functions/equals.cpp

[irenjj]

Can I take this?

[vdimir]

@irenjj

Sure, it'll be really appreciated. If you have any questions during implementation, feel free to reach out.