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https://issues.apache.org/jira/browse/HIVE-16274?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15939290#comment-15939290
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Jason Dere commented on HIVE-16274:
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Unfortunately this scales the nDV of all column in the same way, which makes
the nDV look high for all columns. If the column has a min/max range, or data
type limits, could we at least bound it by those?
The point of having this was because the density function estimate does not
work well in some cases. Longer term it might be nice to have some improvements
in that area as well.
> Support tuning of NDV of columns using lower/upper bounds
> ---------------------------------------------------------
>
> Key: HIVE-16274
> URL: https://issues.apache.org/jira/browse/HIVE-16274
> Project: Hive
> Issue Type: Bug
> Reporter: Pengcheng Xiong
> Assignee: Pengcheng Xiong
> Attachments: HIVE-16274.01.patch
>
>
> For partitioned tables, the distinct value (nDV) estimate for a column is by
> default set to the largest nDV value in any of the partitions being
> considered, which is a lower bound on the nDV estimate.
> This provides a config setting to allow the estimate to a specified fraction
> (0.0 - 1.0) of the higher bound on the nDV estimate (the sum of all the nDVs
> in all partitions).
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