[jira] [Created] (ARROW-5917) [Java] Redesign the dictionary encoder

["Liya Fan (JIRA)" <ji...@apache.org>]

Liya Fan created ARROW-5917:
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             Summary: [Java] Redesign the dictionary encoder
                 Key: ARROW-5917
                 URL: https://issues.apache.org/jira/browse/ARROW-5917
             Project: Apache Arrow
          Issue Type: New Feature
          Components: Java
            Reporter: Liya Fan
            Assignee: Liya Fan


The current dictionary encoder implementation (org.apache.arrow.vector.dictionary.DictionaryEncoder) has heavy performance overhead, which prevents it from being useful in practice:
 # There are repeated conversions between Java objects and bytes (e.g. vector.getObject(i)).
 # Unnecessary memory copy (the vector data must be copied to the hash table).
 # The hash table cannot be reused for encoding multiple vectors (other data structure & results cannot be reused either).
 # The output vector should not be created/managed by the encoder (just like in the out-of-place sorter)
 # The hash table requires that the hashCode & equals methods be implemented appropriately, but this is not guaranteed.

We plan to implement a new one in the algorithm module, and gradually deprecate the current one.



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