viirya commented on PR #2915:
URL: https://github.com/apache/arrow-rs/pull/2915#issuecomment-1289598531

   > I think I am missing something. I don't think there is a way to avoid 
copying and attaching back - this is what MutableArrayData does also? I did 
create #1981 a while back to track allowing mutation of arrays, but currently 
there isn't any support for this?
   
   Oh, no, copying is unavoidable. I meant that I need to create an array first 
and take value value from it. I feel that it is not smooth in usage as letting 
MutableArrayData to handle it as the proposed API. Just my preference.
   
   > > Still copying dictionary value array with another MutableArrayData
   > 
   > This is the step I don't understand, what is this operation you are 
implementing here? Why are you using MutableArrayData for this at all?
   
   We reuse array allocation generally. But for certain operations, we need to 
hold on and wait for all arrays before we can do the operation. So we need to 
copy arrays there otherwise they will be overwritten. But while 
MutableArrayData extends DictionaryArray, it doesn't copy the value array (if 
only one DictionaryArray there).
   
   


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