Suppose the number of values in the dataset is less than the number of buckets in the histogram.
Which value should be returned when producing estimates for col='non-existant-value' ?
Currently, MariaDB's code produces the average. It uses "inclusion assumption" and assumes that the value that is looked up exists in the database. This is not what other databases do.
The deficiency comes from the fact that when one looks at MariaDB's histograms as they were specified in
MDEV-21130 and MDEV-26519, there is no obvious way to tell whether the histogram has an exhaustive list of all values encountered in the table or not.
(Technically, if one walks through the whole histogram and only sees buckets with ndv=1, this will guarantee that the histogram bucket endpoints are all the values that were encountered... but I won't count this as "obvious").
and for various kinds of WHERE clause we get this:
Apparently, MariaDB's assumption differs from that of other databases' and is worse...