The histogram is stored as a JSON document:

The distribution of values can be found in the buckets array of the JSON document. In the above case, the histogram type is singleton. That means that each bucket contains the frequency of a single value. For the other type of histogram, equi-height, each bucket will contain the minimum and maximum value for the range covered by the bucket. The frequency value recorded, is the cumulative frequency. That is, it gives the frequency of values smaller than the maximum value of the bucket. In the example above, 64.27% of the values in the l_linenumber column is less than or equal to 3.

As discussed in my FOSDEM presentation, string values are encoded in the histogram. At the time of the presentation, using MySQL 8.0.11, it was a bit complicated to decode these string values. However, from MySQl 8.0.12 on, this has become simpler. As stated in the release notes for MySQL 8.0.12:

Running the above query on my DBT3 database, I get the following result:

The above gives the cumulative frequencies. Normally, I would rather want to see the actual frequencies of each value, and to get that I will need to subtract the value of the previous row. We can use a window function to do that: