Michal Nowakiewicz created ARROW-12725:
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Summary: [C++][Compute] GroupBy: improve performance by encoding
keys in row format only when they are inserted into hash table
Key: ARROW-12725
URL: https://issues.apache.org/jira/browse/ARROW-12725
Project: Apache Arrow
Issue Type: Improvement
Components: C++
Affects Versions: 4.0.0
Reporter: Michal Nowakiewicz
Fix For: 5.0.0
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.
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