cloud-fan commented on issue #26150: [SPARK-29501][SQL] 
SupportsPushDownRequiredColumns should report pruned schema immediately
URL: https://github.com/apache/spark/pull/26150#issuecomment-544375957
 
 
   > The read schema may be impacted by filters. For example, if I project 
column a and filter by column b then the read schema can be (a, b) or just a. 
So the read schema can't be fully determined when the method to push the 
projection is called.
   
   In fact Spark always do column pruning at the end, so this can be determined 
then `pushRequiredColumns` is called.

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