test=*# select * from (select *, row_number() over(partition by h1 order by u1) from edewolf) as foo where row_number <= 5;
h1 | u1 | name | row_number
----+----+-------------+------------
1 | 1 | Produkt 1 1 | 1
1 | 2 | Produkt 1 2 | 2
1 | 3 | Produkt 1 3 | 3
1 | 4 | Produkt 1 4 | 4
...
test=*# select version();
version
----------------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 13beta1 (Ubuntu 13~beta1-1.pgdg18.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0, 64-bit
(1 row)