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Posted to issues-all@impala.apache.org by "Tim Armstrong (Jira)" <ji...@apache.org> on 2020/01/30 01:09:00 UTC

[jira] [Assigned] (IMPALA-8759) Use double precision for HLL

     [ https://issues.apache.org/jira/browse/IMPALA-8759?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Tim Armstrong reassigned IMPALA-8759:
-------------------------------------

    Assignee: Wenzhe Zhou

> Use double precision for HLL
> ----------------------------
>
>                 Key: IMPALA-8759
>                 URL: https://issues.apache.org/jira/browse/IMPALA-8759
>             Project: IMPALA
>          Issue Type: Improvement
>          Components: Backend
>    Affects Versions: Impala 3.2.0
>            Reporter: Peter Ebert
>            Assignee: Wenzhe Zhou
>            Priority: Major
>              Labels: perf, ramp-up
>
> For /be/src/exprs/aggregate-functions-ir.cc the finalize function uses a float which is only capable of 6-9 digits of precision.  More accurate estimates for larger cardinalities (beyond 999,999) should be possible with double precision.  Another c++ implementation uses double as well [https://github.com/dialtr/libcount]



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