height balanced histogram - cheeyoung/sqlplus-public GitHub Wiki
11.7 Height-Balanced Histograms (Legacy)
In a legacy height-balanced histogram, column values are divided into buckets so that each bucket contains approximately the same number of rows.
For example, if you have 99 coins to distribute among 4 buckets, each bucket contains about 25 coins. The histogram shows where the endpoints fall in the range of values.