Github user wzhfy commented on the issue:
https://github.com/apache/spark/pull/19406
@srowen Before the change, the answer in the test case is `2, 2, 2`.
Based on the code before the change, percentile_approx would never return
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.
> why this method has to use relativeError
`QuantileSummaries` is a sampled data structure, `relativeError` is used to
compute `targeError`, which decides the error bound of the target rank
(https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/QuantileSummaries.scala#L204).
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