DictionaryArray::values() clones the underlying ArrayRef

[tustvold]

The current implementation of DictionaryArray::values() is

/// Returns a reference to the dictionary values array
pub fn values(&self) -> ArrayRef {
    self.values.clone()
}

Aside from a redundant atomic increment on many codepaths, this prevents borrows from propagating correctly, making it challenging to borrow values out of a DictionaryArray.

For example consider a code snippet that wishes to return the first value from a DictionaryArray

fn test(array: &DictionaryArray<Int32Type>) -> &str {
    array.values().as_any().downcast_ref::<StringArray>().unwrap().value(0)
}

Currently this won't compile because the value returned by values() is a stack local temporary. Given one of the motivators of using dictionaries is to avoid redundant copying of the values, this seems unfortunate.

If instead the signature was

/// Returns a reference to the dictionary values array
pub fn values(&self) -> &ArrayRef {
    &self.values
}

The test function above would compile.

Another possibility would be to create a new method, e.g. values_ref(&self) but this seems like a bit of a hack, and returning &ArrayRef would make DictionaryArray consistent with RecordBatch::column.