Zhenhua Wang commented on SPARK-17074:

Well, I've got stuck here for a few days. I went through the QuantileSummaries 
paper and its code in Spark, and I still don't have any clue how to implement 
the second method and get its bounds.
So I decide to adopt the first method for now, such that it won't block our 
progress on CBO work. We can implement the other one in the future.
A PR for a new agg function for string histogram is already sent. I'll start to 
work on this one today and send a pr in the following days. Thanks!

> generate histogram information for column
> -----------------------------------------
>                 Key: SPARK-17074
>                 URL: https://issues.apache.org/jira/browse/SPARK-17074
>             Project: Spark
>          Issue Type: Sub-task
>          Components: Optimizer
>    Affects Versions: 2.0.0
>            Reporter: Ron Hu
> We support two kinds of histograms: 
> -     Equi-width histogram: We have a fixed width for each column interval in 
> the histogram.  The height of a histogram represents the frequency for those 
> column values in a specific interval.  For this kind of histogram, its height 
> varies for different column intervals. We use the equi-width histogram when 
> the number of distinct values is less than 254.
> -     Equi-height histogram: For this histogram, the width of column interval 
> varies.  The heights of all column intervals are the same.  The equi-height 
> histogram is effective in handling skewed data distribution. We use the equi- 
> height histogram when the number of distinct values is equal to or greater 
> than 254.  

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