ulysses-you opened a new pull request, #36180:
URL: https://github.com/apache/spark/pull/36180
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### What changes were proposed in this pull request?
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Enhance the selection of `SortMergeJoin` streamed side and buffered side in
`JoinSelection`, so that we can get a buffered side for better performance.
### Why are the changes needed?
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For an inner join type SortMergeJoin, it always uses the left side as
streamed side and right side as buffered side.
Accoirding to the implementaion of SortMergeJoin, we expect the buffered
side to be:
- smaller than streamed side
- has less duplicate data
We do not know whether the join will be SortMergeJoin at logical phase, so
it should do this selection at physcial phase.
For example:
```sql
SELECT * FROM (
SELECT c1 FROM a GROUP BY c1
) t1 JOIN t2 ON t1.c1 = t2.c2
```
We can make t1 as the buffered side if its statistics size less than t2.
### Does this PR introduce _any_ user-facing change?
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The plan may be changed and improve the performance.
### How was this patch tested?
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add test
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