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From "Juliusz Sompolski (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SPARK-24013) ApproximatePercentile grinds to a halt on sorted input.
Date Wed, 18 Apr 2018 13:16:00 GMT

    [ https://issues.apache.org/jira/browse/SPARK-24013?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16442477#comment-16442477
] 

Juliusz Sompolski commented on SPARK-24013:
-------------------------------------------

This hits when trying to create histogram statistics (SPARK-21975) on columns like monotonically
increasing id - histograms cannot be created in reasonable time.

> ApproximatePercentile grinds to a halt on sorted input.
> -------------------------------------------------------
>
>                 Key: SPARK-24013
>                 URL: https://issues.apache.org/jira/browse/SPARK-24013
>             Project: Spark
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 2.3.0
>            Reporter: Juliusz Sompolski
>            Priority: Major
>
> Running
> {code}
> sql("select approx_percentile(rid, array(0.1)) from (select rand() as rid from range(10000000))").collect()
> {code}
> takes 7 seconds, while
> {code}
> sql("select approx_percentile(id, array(0.1)) from range(10000000)").collect()
> {code}
> grinds to a halt - processes the first million rows quickly, and then slows down to a
few thousands rows / second (4m rows processed after 20 minutes).
> Thread dumps show that it spends time in QuantileSummary.compress.
> Seems it hits some edge case inefficiency when dealing with sorted data?



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