westonpace commented on PR #13799:
URL: https://github.com/apache/arrow/pull/13799#issuecomment-1209587810

   Yes, scanner builder is on its way out, I hope, as part of #13782 (well, 
probably a follow-up).  At the moment it still serves a slight purpose in that 
the projection option is a little hard to specify and it is something of a 
thorn when it comes to augmented fields.
   
   I also agree with your other point.  We spent considerable effort at one 
point making various things look like a dataset because datasets were the 
primary interface to the compute engine (e.g. filtering & projection).  The 
record batch reader example is a good example.  I'd even go so far as to say 
the InMemoryDataset is probably superfluous and a better option in the future 
would be a "table_source" node.  The scanner should be reserved for the case 
where you have multiple sources of data, with the same (or devolved versions of 
the same) schema.
   
   All that being said, I don't think readahead is going away.  However, in the 
near future (again, #13782) I was pondering if we should reframe readahead as 
"roughly how many bytes of data should the scanner attempt to read ahead" 
instead of "batch readahead and fragment readahead".
   


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