Histograms: Most-Common-Values histograms

Often, columns have a few values that are very frequent.
For this case, one can use a histogram that is a collection of (value, frequency) pairs.

There are [approximate] algorithms that allow to find most common values while using a limited amount of memory and/or basing on sample of the table.

Another important property is that most-common-value collection/storage can be generalized to tuples of multiple columns.