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From "Ashutosh Chauhan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-4435) Column stats: Distinct value estimator should use hash functions that are pairwise independent
Date Mon, 03 Jun 2013 21:26:20 GMT

    [ https://issues.apache.org/jira/browse/HIVE-4435?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13673595#comment-13673595
] 

Ashutosh Chauhan commented on HIVE-4435:
----------------------------------------

Sorry for the delay. +1 Will commit if tests pass.
                
> Column stats: Distinct value estimator should use hash functions that are pairwise independent
> ----------------------------------------------------------------------------------------------
>
>                 Key: HIVE-4435
>                 URL: https://issues.apache.org/jira/browse/HIVE-4435
>             Project: Hive
>          Issue Type: Bug
>          Components: Statistics
>    Affects Versions: 0.10.0
>            Reporter: Shreepadma Venugopalan
>            Assignee: Shreepadma Venugopalan
>         Attachments: chart_1(1).png, HIVE-4435.1.patch
>
>
> The current implementation of Flajolet-Martin estimator to estimate the number of distinct
values doesn't use hash functions that are pairwise independent. This is problematic because
the input values don't distribute uniformly. When run on large TPC-H data sets, this leads
to a huge discrepancy for primary key columns. Primary key columns are typically a monotonically
increasing sequence.

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