stxkind only explain two elements. left two unexplained.

[Jian He]

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.