Rich-T-kid commented on PR #21765:
URL: https://github.com/apache/datafusion/pull/21765#issuecomment-4676981749

   When arrow-rs reads RLE-encoded strings from a Parquet file, it decodes them 
into dictionary arrays. These flow through operators like any other array. For 
operators like `AggregateExec`, the dictionary gets expanded during grouping: 
the `GroupValuesRows` grouping implementation uses `RowConverter`, which 
expands dictionaries when converting columns to their row representation. 
([source](https://github.com/apache/arrow-rs/blob/301eb26bb92362e531bd6c39980292ebcae8e8aa/arrow-row/src/lib.rs#L2060))
   
   


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