[jira] [Commented] (IMPALA-9939) Fix Hive interop for HLL with STRING types

["Tim Armstrong (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/IMPALA-9939?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17154792#comment-17154792 ] 

Tim Armstrong commented on IMPALA-9939:
---------------------------------------

+1 on adopting normalised utf-8 as the standard representation. Most file formats use utf-8 as the internal representation (Parquet included) so hashing anything else would limit how much it could be optimised in future.


> Fix Hive interop for HLL with STRING types
> ------------------------------------------
>
>                 Key: IMPALA-9939
>                 URL: https://issues.apache.org/jira/browse/IMPALA-9939
>             Project: IMPALA
>          Issue Type: Bug
>          Components: Backend
>            Reporter: Csaba Ringhofer
>            Priority: Major
>
> It turned out that Impala hashes STRINGs differently than Hive.
> Impala's implementation simply hashes the original byte array (e.g. a UTF-8 encoded string), while Hive hashes the UTF-16 encoded char array behind java strings. If the STRING is cast to BINARY in Hive (e.g. ds_hll_sketch(cast(s as binary)) ), then it is interoperable with Impala's current implementation.
> I am not sure how to proceed - we could UTF-16 encode the strings in Impala before hashing, but this would be pretty slow, and I think that Hive actually could be also faster if it would hash UTF-8 arrays - as STRINGs are stored as org.apache.hadoop.io.Text, they are currently UTF-8 decoded to java string first and could be hashed directly without any conversion. This would break compatibility with existing Hive produced sketches though.



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