[C++][Documentation] Explicit documentation of null slot interpretation

[asfimport]

To my knowledge, there isn't explicit documentation on how null slots in an array should be interpreted. SQL uses Kleene logic, wherein a null is explicitly an unknown rather than a special value. This yields for example (null AND false) -> false, since (x AND false) -> false for all possible values of x. This is also the behavior of Gandiva's boolean expressions.

By contrast the boolean kernels implement something closer to the behavior of NaN: (null AND false) -> null. I think this is simply an error in the boolean kernels but in any case I think explicit documentation should be added to prevent future confusion.

Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz

Related issues:

• [C++] Add ResolveNullOptions to Logical kernels (is fixed by)

PRs and other links:

• GitHub Pull Request #5231

_{Note: This issue was originally created as ARROW-6386. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
If the current boolean kernel behavior is necessary for some use cases an option struct could be added to make this configurable

[asfimport]

Wes McKinney / @wesm:
Do you want to document this as a comment in the C++ kernels code? I don't think any changes need to be made to the columnar format documentation

[asfimport]

Ben Kietzman / @bkietz:
Interpretation of nulls will be left open for individual kernels. #5771 adds explicit Kleene logic overloads of the boolean kernels, allowing SQL compatible behavior in dataset filter expressions.