Github user facaiy commented on the issue:
https://github.com/apache/spark/pull/17556
For a (train) sample of continuous series, say {x0, x1, x2, x3, ..., x100}.
Now spark select quantile as split point.
Suppose 10-quantiles is used, and x2 is 1st quantile, and x10 is 2nd
quantile. It's believed that P(x < x2) ~= P(x2 < x < x10). However, x2 is not
perfect. As the data is continuous, there exits one point z is the real point
who satisfy P(x < z) == P(z < x < x10).
And it's reasonable that averaged midpoint between x2 and x3 is more
appropriate, in my option.
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