FrankChen021 commented on PR #12745:
URL: https://github.com/apache/druid/pull/12745#issuecomment-1175690119

   @gianm I think introducing columnar-based format is very helpful. 
   
   One thing I don't understand from intuition is that why the row-based format 
improves the final query performance because this kind of data format 
converting on large amount of data could be time-consuming.
   
   In ClickHouse, columnar format is always used to transfer intermidiate 
sub-query result between servers, and when the final result is returned to 
client in row-based format, we find that converting from columnar formart to 
row-based format and serializing each field into text format is very time 
consuming.
   
   Could you show me where the columnar data format is converted into row-based 
format? 


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