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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