zhmin opened a new pull request, #40567:
URL: https://github.com/apache/spark/pull/40567
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### What changes were proposed in this pull request?
We indroduce a new idea to optimize exchange plan when union spark plan
output partitoning can't match parent plan's required distribution.
1. First introduce a new RDD, it consists of parent rdds that has the same
partition size. The ith parttition corresponds to ith partition of each parent
rdd.
2. Then push the required distribution to union plan's children. If any
child output partitioning matches the required distribution , we can reduce
this child shuffle operation.
### Why are the changes needed?
Union plan does not take full advantage of children plan output
partitionings when output partitoning can't match parent plan's required
distribution.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
org.apache.spark.sql.execution.PlannerSuite
org.apache.spark.sql.execution.exchange.UnionZipRDDSuite
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