[jira] [Work started] (HIVE-29300) Wrong estimation for num rows in EXPLAIN with histogram statistics

["Thomas Rebele (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HIVE-29300?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Work on HIVE-29300 started by Thomas Rebele.
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> Wrong estimation for num rows in EXPLAIN with histogram statistics
> ------------------------------------------------------------------
>
>                 Key: HIVE-29300
>                 URL: https://issues.apache.org/jira/browse/HIVE-29300
>             Project: Hive
>          Issue Type: Bug
>          Components: Statistics
>    Affects Versions: 4.1.0
>            Reporter: Thomas Rebele
>            Assignee: Thomas Rebele
>            Priority: Minor
>         Attachments: stats_histogram2.q, stats_histogram2.q.out
>
>
> Given a query {{{}SELECT 1 FROM sh2a WHERE k1 < 10 AND k2 < 250{}}}, with a selectivity of 0.02 for {{k1 < 10}} and 0.5 for {{{}k2 < 250{}}}, the combined selectivity should be 0.01 resulting in selecting 5 rows of the 500 rows of the table.
> If we activate histograms, the combined selectivity is estimated only with the last clause as approx 0.5, so estimating that 247 rows of 500 rows are selected.
> Reason: {{StatsRulesProcFactory.FilterStatsRule#evaluateComparatorWithHistogram}} needs to estimate the selectivity of the condition and multiply it by currNumRows.



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