stxkind only explain two elements. left two unexplained.

PostgreSQL: Documentation: 10: 51.51. pg_statistic_ext
<https://www.postgresql.org/docs/10/catalog-pg-statistic-ext.html>

> stxkind : An array containing codes for the enabled statistics kinds;
> valid values are: d for n-distinct statistics, f for functional
> dependency statistics

select distinct stxkind from pg_statistic_ext ;
return:

stxkind
> ---------
> {d}
> {m}
> {f}
> {e}
> (4 rows)

I guess *e *for expression statistics. Since I create the following
statistics object.

create statistics flight_expr1 on(
extract
(month from scheduled_departure at time zone 'Asia/Kolkata')
) from flights;

select stxkind from pg_statistic_ext where stxname = 'flight_expr';

and it return *e.*
But I am not sure what does *m *mean?

It would be better to full explain all these four elements in the doc.