\set ECHO none
select count_distinct(x) over (order by x rows between unbounded preceding and current row)
  from test_data_1_20;
 count_distinct 
----------------
              1
              2
              3
              4
              5
              6
              7
              8
              9
             10
             11
             12
             13
             14
             15
             16
             17
             18
             19
             20
(20 rows)

select count_distinct(x) over (order by x rows between 10 preceding and 10 following)
  from test_data_1_25;
 count_distinct 
----------------
             11
             12
             13
             14
             15
             16
             17
             18
             19
             20
             21
             21
             21
             21
             21
             20
             19
             18
             17
             16
             15
             14
             13
             12
             11
(25 rows)

ROLLBACK;