cloud-fan commented on issue #25280: [SPARK-28548][SQL] explain() shows wrong 
result for persisted DataFrames after some operations
URL: https://github.com/apache/spark/pull/25280#issuecomment-527012092
 
 
   This is not a correctness issue. This is just an undocumented property "a 
dataframe can't change its physical plan once the physical plan is 
materialized" causing confusions.
   
   After so many days, I have a fresh idea now: when `df.cache` is called, 
reset the `cachedPlan`, `optimizedPlan` and physical plans. We can implement 
the lazy evaluation manually to support it instead of relying on Scals lazy 
val.  

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