cloud-fan opened a new pull request, #42315:
URL: https://github.com/apache/spark/pull/42315
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### What changes were proposed in this pull request?
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We have a long-standing tricky optimization in `Dataset.union`, which
invokes the optimizer rule `CombineUnions` to pre-optimize the analyzed plan.
This is to avoid too large analyzed plan for a specific dataframe query pattern
`df1.union(df2).union(df3).union...`.
This tricky optimization is designed to break dataframe caching, but we
thought it was fine as people usually won't cache the intermediate dataframe in
a union chain. However, `CombineUnions` gets improved from time to time and now
it can optimize a wide range of Union patterns. Now it's possible that people
union two dataframe, do something with `select`, and cache it. Then the
dataframe is unioned again with other dataframes and people expect the df cache
to work. However the cache won't work due to the tricky optimization in
`Dataset.union`.
This PR updates `Dataset.union` to only combine adjacent Unions to match the
original purpose.
### Why are the changes needed?
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Fix perf regression due to breaking df caching
### Does this PR introduce _any_ user-facing change?
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no
### How was this patch tested?
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new test
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