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Posted to issues-all@impala.apache.org by "Peter Ebert (Jira)" <ji...@apache.org> on 2019/09/05 17:57:00 UTC
[jira] [Commented] (IMPALA-8759) Use double precision for HLL
[ https://issues.apache.org/jira/browse/IMPALA-8759?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16923660#comment-16923660 ]
Peter Ebert commented on IMPALA-8759:
-------------------------------------
Yes I am sure that was a thought choice, however even if it's 2x faster with 32 bit vs 64 bit, since it's a single loop at the end of the finalize the cost would be fixed and doubt it would add even a millisecond, more probably nanoseconds.
I will say though I don't have much to prove that it is significantly better in estimation. I did some +very+ basic testing (used tpc-ds generated data) and although it was more accurate in my very non-scientific tests it was in the 7th digit and my data set wasn't large enough for that to be significant. Definitely needs more testing but initially it seems like not a large difference.
> 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
> 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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