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From "Zhenhua Wang (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (SPARK-17074) generate histogram information for column
Date Sat, 22 Oct 2016 12:52:58 GMT

    [ https://issues.apache.org/jira/browse/SPARK-17074?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15570366#comment-15570366
] 

Zhenhua Wang edited comment on SPARK-17074 at 10/22/16 12:52 PM:
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Well, I've got stuck here for a few days. I went through the QuantileSummaries paper and our
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 counting ndv's of multiple intervals is already sent.


was (Author: zenwzh):
Well, I've got stuck here for a few days. I went through the QuantileSummaries paper and our
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 (equi-width) 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.  
> We first use [SPARK-18000] and [SPARK-17881] to compute equi-width histograms (for both
numeric and string types) or endpoints of equi-height histograms (for numeric type only).
Then, if we get endpoints of a equi-height histogram, we need to compute ndv's between those
endpoints by [SPARK-17997] to form the equi-height histogram.
> This Jira incorporates three Jiras mentioned above to support needed aggregation functions.
We need to resolve them before this one.



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