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

    https://github.com/apache/spark/pull/16578#discussion_r139331293
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetReadSupport.scala
 ---
    @@ -63,9 +74,22 @@ private[parquet] class ParquetReadSupport extends 
ReadSupport[UnsafeRow] with Lo
           StructType.fromString(schemaString)
         }
     
    -    val parquetRequestedSchema =
    +    val clippedParquetSchema =
           ParquetReadSupport.clipParquetSchema(context.getFileSchema, 
catalystRequestedSchema)
     
    +    val parquetRequestedSchema = if (parquetMrCompatibility) {
    +      // Parquet-mr will throw an exception if we try to read a superset 
of the file's schema.
    +      // Therefore, we intersect our clipped schema with the underlying 
file's schema
    --- End diff --
    
    `clipParquetSchema` can add column paths only exist in catalyst schema into 
parquet schema. I think those columns are useful but looks like the 
`intersectParquetGroups` will remove them again?


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