Github user kiszk commented on the issue:
https://github.com/apache/spark/pull/13704
Regarding the plan tree printout (I removed my debug information), the
```Cast``` performs conversion from ```UnsafeArrayData``` to
```GenericArrayData``` since the target type of ```Cast``` is
```ArrayType<DoubleType>```. Since ```Cast``` shows ```dataType. simpleType```
of the target type, it is shown as ```array<double>```.
Regarding the generated code, we seems to be on the same page. What you
said is not done in ```Cast``` now, and what I did. IIUC, current ```Cast```
[code generation for
array](https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala#L828)
seems to be conservative. Current goal of cast code generation is to create
```GenericArrayData``` object. This code generation always creates
```Object[]``` and assign values into each ```Object[]``` element. Then, the
generated code passes ```Object[]``` to the constructor of
```GenericArrayData```. As you pointed out, if code generation takes care of
nullability, it can avoid to create ```Object[]```. Unfortunately, it is not
done in [the current code]([code generation for
array](https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala#L828)
(Lines 828-862).
The code generated by this PR can also use specialized
```GenericArrayData``` implemented by
https://github.com/apache/spark/pull/13758.
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