Github user viirya commented on a diff in the pull request:

    https://github.com/apache/spark/pull/16578#discussion_r149880058
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala ---
    @@ -961,6 +961,15 @@ object SQLConf {
         .booleanConf
         .createWithDefault(true)
     
    +  val NESTED_SCHEMA_PRUNING_ENABLED =
    +    buildConf("spark.sql.nestedSchemaPruning.enabled")
    +      .internal()
    +      .doc("Prune nested fields from a logical relation's output which are 
unnecessary in " +
    +        "satisfying a query. This optimization allows columnar file format 
readers to avoid " +
    +        "reading unnecessary nested column data.")
    --- End diff --
    
    Please also mention this is only applied on Parquet data source for now.


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