[jira] [Created] (SPARK-22208) Improve percentile_approx by not rounding up targetError and starting from index 0

["Zhenhua Wang (JIRA)" <ji...@apache.org>]

Zhenhua Wang created SPARK-22208:
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             Summary: Improve percentile_approx by not rounding up targetError and starting from index 0
                 Key: SPARK-22208
                 URL: https://issues.apache.org/jira/browse/SPARK-22208
             Project: Spark
          Issue Type: Improvement
          Components: SQL
    Affects Versions: 2.3.0
            Reporter: Zhenhua Wang


percentile_approx never returns the first element when percentile is in (relativeError, 1/N], where relativeError default is 1/10000, and N is the total number of elements. But ideally, percentiles in [0, 1/N] should all return the first element as the answer.

For example, given input data 1 to 10, if a user queries 10% (or even less) percentile, it should return 1, because the first value 1 already reaches 10%. Currently it returns 2.

Based on the paper, targetError is not rounded up, and searching index should start from 0 instead of 1. By following the paper, we should be able to fix the cases mentioned above.



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