Dandandan commented on a change in pull request #55:
URL: https://github.com/apache/arrow-datafusion/pull/55#discussion_r619873424
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File path: datafusion/src/physical_plan/hash_join.rs
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@@ -891,6 +898,36 @@ impl Stream for HashJoinStream {
}
Some(result)
}
+ // If maybe_batch is None and num_output_rows is 0, that means
right side batch was
+ // empty and has been coalesced to None. Fill right side with
Null if preserve_left
+ // is true.
+ None if self.preserve_left && self.num_output_rows == 0 => {
Review comment:
I think this partially resolves a more general issue with the left join,
which is that it doesn't keep track of unmatched left rows across batches.
https://issues.apache.org/jira/browse/ARROW-10971
Maybe we can add a TODO here / issue that we should generalize this to
produce rows that were not matched.
This looks like a great start for that 👍
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