Weston Pace created ARROW-12386:
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Summary: [C++] Support file parallelism in AsyncScanner
Key: ARROW-12386
URL: https://issues.apache.org/jira/browse/ARROW-12386
Project: Apache Arrow
Issue Type: Improvement
Components: C++
Reporter: Weston Pace
Assignee: Weston Pace
Whether we pull from files in parallel or not is controlled by how we merge the
batch streams in `AsyncScanner::ScanBatchesUnorderedAsync`. Currently we are
relying on `MakeConcatenatedGenerator` which is incorrect. This is needed
because `MakeMergedGenerator` pulls from its source (an `EnumeratingGenerator`)
in an async reentrant fashion. `MakeMergedGenerator` should not do this. If
some kind of readahead is truly necessary there then use
`MakeReadaheadGenerator`.
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