c21 commented on pull request #31958:
URL: https://github.com/apache/spark/pull/31958#issuecomment-810970640


   > For orc files without nested schema, do we observe perf regression after 
this PR?
   
   @cloud-fan - in theory here the difference for non-nested schema:
   
   Before this PR:
   
   ```
   OrcColumnVector -> org.apache.spark.sql.vectorized.ColumnVector
   ```
   
   After this PR:
   
   ```
   OrcAtomicColumnVector -> OrcColumnVector -> 
org.apache.spark.sql.vectorized.ColumnVector
   ```
   
   The only overhead introduced here is one more layer in classes, and might be 
some overhead for virtual function call and class loading.
   
   Tested on same type of machine in AWS EC2 for all ORC reader benchmarks: 
[OrcReadBenchmark-results.txt](https://github.com/apache/spark/blob/master/sql/hive/benchmarks/OrcReadBenchmark-results.txt)
 and 
[DataSourceReadBenchmark-results.txt](https://github.com/apache/spark/blob/master/sql/core/benchmarks/DataSourceReadBenchmark-results.txt)
 (java 8 here). Do not see regression compared ORC vectorized reader and other 
ORC readers.
   
   Results:
   
   
[OrcReadBenchmark-results.txt](https://gist.github.com/c21/978bd38abb781af83ec19cec80000759)
   
[DataSourceReadBenchmark-results.txt](https://gist.github.com/c21/62391f8cfdaf4f15acac0ee168e3b86b)
   
   Machine:
   
   ```
   Amazon AWS EC2
   type: r3.xlarge
   region: us-west-2 (Oregon)
   OS: Linux
   ```


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