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Posted to github@arrow.apache.org by GitBox <gi...@apache.org> on 2022/06/22 19:57:14 UTC

[GitHub] [arrow-rs] viirya opened a new issue, #1923: Disallow cast from other datatypes to NullType

viirya opened a new issue, #1923:
URL: https://github.com/apache/arrow-rs/issues/1923

**Describe the bug**

We allow casting from other datatypes to NullType (see `can_cast_types`). I suspect that this is a mistake.

For example, Spark doesn't allow casting from any datatypes to NullType.

I also checked C++ Cast kernels. Mostly the datatypes cannot cast to NullType (e.g. https://github.com/apache/arrow/blob/53752adc6b81166cd4ee7db5a819494042f29197/cpp/src/arrow/compute/kernels/scalar_cast_test.cc#L187-L211). I said "mostly" because I don't see explicit tests for other cases like Decimal, but I suspect that it is not allowed but just lack of test coverage.

I don't see there is cast kernel in C++ for casting to NullType except for a special case (`dict<null> -> null`): https://github.com/apache/arrow/blob/53752adc6b81166cd4ee7db5a819494042f29197/cpp/src/arrow/compute/kernels/scalar_cast_numeric.cc#L748-L751. Even for that, there is a comment questioning about it.

**To Reproduce**
Steps to reproduce the behavior:

**Expected behavior**

Cast kernel should not allow casting from any datatypes to NullType.

**Additional context**
Add any other context about the problem here.

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[GitHub] [arrow-rs] liukun4515 commented on issue #1923: Disallow cast from other datatypes to NullType

Posted by GitBox <gi...@apache.org>.

liukun4515 commented on issue #1923:
URL: https://github.com/apache/arrow-rs/issues/1923#issuecomment-1165079647

   The result of discussion: https://github.com/apache/arrow-rs/pull/1922#discussion_r905436410


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[GitHub] [arrow-rs] liukun4515 closed issue #1923: Disallow cast from other datatypes to NullType

Posted by GitBox <gi...@apache.org>.

liukun4515 closed issue #1923: Disallow cast from other datatypes to NullType
URL: https://github.com/apache/arrow-rs/issues/1923


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[GitHub] [arrow-rs] liukun4515 commented on issue #1923: Disallow cast from other datatypes to NullType

Posted by GitBox <gi...@apache.org>.

liukun4515 commented on issue #1923:
URL: https://github.com/apache/arrow-rs/issues/1923#issuecomment-1165273471

   I will give a follow-up pr to remove casting other data type to Null Data type.


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