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

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.3.0
• Fix Version/s: 2.3.0
• Component/s: SQL
• Labels: releasenotes
• Assignee: Zhenhua Wang
• Reporter: Zhenhua Wang
• Created: 05/Oct/17 15:42
• Updated: 21/Jan/18 21:03
• Resolved: 11/Oct/17 07:19

Description

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.

Issue Links

• is duplicated by: SPARK-22179 percentile_approx should choose the first element if it already reaches the percentage
• links to: [Github] Pull Request #19438 (wzhfy)