Github user jaltekruse commented on the issue:
https://github.com/apache/drill/pull/546
The original behavior was not by design, it was a bug. I would just detect
the case where a scalar is being flattened and throw an appropriate user facing
message like "Flatten does not support inputs of non-list values".
The tests you have included here don't check their results. I would also
suggest adding a test for the new negative case, checking for the appropriate
message.
If you want to more precisely control the input to your test, I would
recommend taking a look at the operator unit test system I checked in and
demonstrated at the hangout last month. This will allow you to write more
targeted tests for this case. Feel free to ask questions if the usage is not
clear from the examples in this class.
https://github.com/apache/drill/blob/master/exec/java-exec/src/test/java/org/apache/drill/exec/physical/unit/BasicPhysicalOpUnitTest.java
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