sarutak commented on PR #56595:
URL: https://github.com/apache/spark/pull/56595#issuecomment-4754109661

   Thank you for the feedback, @peter-toth @uros-b.
   Added a test that forces spill by setting 
`spark.sql.sortMergeJoinExec.buffer.in.memory.threshold` and 
`spark.sql.sortMergeJoinExec.buffer.spill.threshold` to `1`. The test covers 
both the no-equi-key path (bufferAllRight) and the equi-key path 
(bufferRightGroup).
   Regarding the I/O concern: the per-left-row generateIterator() re-scan is 
the same pattern as `SortMergeJoinExec` (which also re-iterates bufferedMatches 
for each streamed row). In practice, spill only occurs for extremely skewed 
groups where the alternative was OOM. The typical case stays in-memory.


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