vertexclique commented on pull request #8430:
URL: https://github.com/apache/arrow/pull/8430#issuecomment-714323287


   > (this part is regardless, and could fit in a future PR) You mentioned in 
the commit message that this drastically reduces performance. Did you 
quantified how much was that, or is there any benchmark that you have in mind 
that I can run on my own? I am asking that because that is one of the 
situations where unsafe can be considered based on the risks and maintenance vs 
performance benefits.
   
   Based on my estimations of how I wrote it, it will increase the parsing time 
for long record windows with large nested element size. I don't estimate how 
large will cause it but the main concern here is that refcell's stacked borrow 
and finalizer's dynamic dispatch. This can be decreased by pruning refcell by 
visiting the ticket that I mentioned in there to unify the interfaces. Then we 
will use dynamic dispatch in everywhere. Thou, dynamic dispatch can be slightly 
slower compared to pointer casts(or in-place transmutes) it can solve the 
overhead of refcell. This piece of code is open to revisiting after 
[ARROW-10335](https://issues.apache.org/jira/browse/ARROW-10335).


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