Previous implementation of hash group by converts input ExecBatches to row-oriented format,
then hashes and compares rows as if they were a single column. 
It is more efficient (especially for small number of key columns) to avoid relatively costly 
encoding and instead compute hashes of individual columns in column-oriented format mixing them together, and similarly comparing column-oriented data to row-oriented data in the hash table without converting. 
Encoding only happens for a subset of input rows that are inserted into the hash table - they introduce new groups. 
Keys in hash table remain stored as row-oriented.

Reporter: Michal Nowakiewicz / @michalursa
Assignee: Michal Nowakiewicz / @michalursa

Related issues:

• [C++] Query engine umbrella issue (is a child of)

PRs and other links:

• GitHub Pull Request #10290

_{Note: This issue was originally created as ARROW-12725. Please see the migration documentation for further details.}