[jira] [Resolved] (SPARK-17074) generate equi-height histogram for column

["Wenchen Fan (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SPARK-17074?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Wenchen Fan resolved SPARK-17074.
---------------------------------
       Resolution: Fixed
    Fix Version/s: 2.3.0

Issue resolved by pull request 19479
[https://github.com/apache/spark/pull/19479]

> generate equi-height histogram for column
> -----------------------------------------
>
>                 Key: SPARK-17074
>                 URL: https://issues.apache.org/jira/browse/SPARK-17074
>             Project: Spark
>          Issue Type: Sub-task
>          Components: Optimizer
>    Affects Versions: 2.3.0
>            Reporter: Ron Hu
>             Fix For: 2.3.0
>
>
> Equi-height histogram is effective in handling skewed data distribution.
> For equi-height histogram, the heights of all bins(intervals) are the same. The default number of bins we use is 254.
> Now we use a two-step method to generate an equi-height histogram:
> 1. use percentile_approx to get percentiles (end points of the equi-height bin intervals);
> 2. use a new aggregate function to get distinct counts in each of these bins.
> Note that this method takes two table scans. In the future we may provide other algorithms which need only one table scan.



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