[jira] [Created] (ARROW-6184) [Java] Provide hash table based dictionary encoder

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

Liya Fan created ARROW-6184:
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             Summary: [Java] Provide hash table based dictionary encoder
                 Key: ARROW-6184
                 URL: https://issues.apache.org/jira/browse/ARROW-6184
             Project: Apache Arrow
          Issue Type: New Feature
          Components: Java
            Reporter: Liya Fan
            Assignee: Liya Fan


This is the second part of ARROW-5917. We provide a sort based encoder, as well as a hash table based encoder, to solve the problem with the current dictionary encoder. 

In particular, we solve the following problems with the current encoder:
 # 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.



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