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Posted to dev@hive.apache.org by "Ashutosh Chauhan (JIRA)" <ji...@apache.org> on 2013/06/05 16:16:21 UTC

[jira] [Updated] (HIVE-4435) Column stats: Distinct value estimator should use hash functions that are pairwise independent

     [ https://issues.apache.org/jira/browse/HIVE-4435?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Ashutosh Chauhan updated HIVE-4435:
-----------------------------------

    Affects Version/s: 0.11.0
               Status: Open  (was: Patch Available)

Canceling patch since current patch is resulting in test failures.
                
> 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: 0.11.0, 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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