Refactor SortExec's buffered batches for better code readability #15372
Description
Is your feature request related to a problem or challenge?
Reference #15355 (comment) #15355 (comment)
The high-level execution logic of SortExec is described in https://github.com/apache/datafusion/blob/main/datafusion/physical-plan/src/sorts/sort.rs, now a single field in_mem_batches is used to represent buffered data in different stage: during different time, it can be interpreted as either unordered input batch, or globally sorted batch:
This approach has poor understandability and is also error-prone, see the idea in reference discussion for improvements.
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