Equi-height histogram is effective in handling skewed data distribution.
For equi-height histogram, the heights of all bins(intervals) are the same. The default number of bins we use is 254.
Now we use a two-step method to generate an equi-height histogram:
1. use percentile_approx to get percentiles (end points of the equi-height bin intervals);
2. use a new aggregate function to get distinct counts in each of these bins.
Note that this method takes two table scans. In the future we may provide other algorithms which need only one table scan.