alamb commented on PR #9093:
URL: https://github.com/apache/arrow-rs/pull/9093#issuecomment-3730681145

   > from experiment: Pre-allocation overhead may offset the savings from 
avoiding incremental growth
   
   @lyang24  -- I am not sure that the `num_rows` you are passing in is 
actually the total number of rows which would be output. Looking at the 
internals of the reader, it almost looks like the record_reader doesn't 
currently get told how many records it may get -- however, the higher level 
APIs certainly know the max size (it is the `batch_size`)
   
   Maybe we could pass in the batch size to the reader...


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