Github user wzhfy commented on the issue:
https://github.com/apache/spark/pull/15637
Yes, in our design, equi-width histogram is a seq of single valued bins,
which is used for columns with low cardinality, so that we can get accurate
estimation. When cardinality is high, equi-height histogram is better than
equi-width histogram (with some fixed width), especially in data skew scenario,
where the skewed value would be hidden in a equi-width bin, while in
equi-height histogram, it would occupies many contiguous bins.
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