LantaoJin opened a new pull request #28947:
URL: https://github.com/apache/spark/pull/28947
### What changes were proposed in this pull request?
In the `apply` method of `OptimizeSkewedJoin`, we first match out the
`UnionExec` nodes, then try to optimize their children with current logic.
### Why are the changes needed?
Current, the AQE skew join only supports two tables join such as
```
SMJ
:-Sort
: +-Shuffle
+-Sort
+-Shuffle
```
But if the plan contains a Union, the skew join handling not work:
```
Union
:-SMJ
: :-Sort
: : +-Shuffle
: +-Sort
: +-Shuffle
+-SMJ
: :-Sort
: : +-Shuffle
: +-Sort
+-Shuffle
```
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Add a UT.
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