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https://issues.apache.org/jira/browse/HIVE-20260?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16562086#comment-16562086
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Zoltan Haindrich commented on HIVE-20260:
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I feel that this logic might need to be rethinked at some point...relying the
calculation more on the the column stats - however I'm afraid that won't be
possible since they might not be available all the time...
I've introduced some logic to keep track of the affected columns; this makes it
much better. However...a full test run is needed to see if it causes any
trouble for other queries
https://reviews.apache.org/r/68109/
> NDV of a column shouldn't be scaled when row count is changed by filter on
> another column
> -----------------------------------------------------------------------------------------
>
> Key: HIVE-20260
> URL: https://issues.apache.org/jira/browse/HIVE-20260
> Project: Hive
> Issue Type: Improvement
> Components: Statistics
> Reporter: Ashutosh Chauhan
> Assignee: Zoltan Haindrich
> Priority: Major
> Attachments: HIVE-20260.01wip01.patch, HIVE-20260.01wip02.patch
>
>
> HIVE-17465 introduced progressive scaling of rowcounts in presence of
> multiple filters. HIVE-19500 improved on that by also scaling col stats (NDV)
> in such scenario. However, it should pay attention to column used in filter
> expression and not scale for all filters. eg.,
> consider filter a = 1 and b = 2 ndv of column b should not be scaled down by
> row count changes caused by a = 1
> Other way to say this that ndv of a particular column should be updated at
> the end of computation of row count for that operator.
> Here are the possible cases where our estimates can be accurate (or close to)
> {code}
> case 1 - (d_year = 2001 and d_moy=1)
> case 2 - (d_year = 2001 and d_year IN (2001, 2002))
> case 3 - (d_year = 2001 and d_moy = 1 and d_dom = 1)
> case 4 - (d_date IN ('1999-01-02', '1999-01-02'))
> case 5 - (d_date = '1999-01-01')
> {code}
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