[GitHub] [arrow] lidavidm edited a comment on pull request #10145: ARROW-12522: [C++] Add ReadRangeCache::WaitFor

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lidavidm edited a comment on pull request #10145:
URL: https://github.com/apache/arrow/pull/10145#issuecomment-827784250


   > I wonder if, given a bunch of small record batches, we might sometimes want to coalesce across record batches. I think the current design preempts that. Although I think there would be more challenges than just this tool to tackle that problem.
   
   So overall, the use pattern for this class is:
   
   1. `Cache()` all byte ranges you expect to read in the future, in the granularity that you expect to read them. So you'd call `Cache` for every record batch (IPC), or for every column chunk (Parquet).
   2. `WaitFor()` the ranges that you need. For IPC, this would again be one record batch; for Parquet, this would be one row group's worth of column chunks. This can be done in parallel/reentrantly and is why we need the lock in the lazy variant.
   3. `Read` the ranges that you need.
   
   Since all the byte ranges are given up front, you do get coalescing across record batches/column chunks.


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