如果名称是唯一的

select users.name,count(*)
from messages join users on messages.created_by=users.uuid
where messages.created_at>=now()-'24h'::interval
group by 1 order by 2 desc limit 10;

字符串
否则

select users.name,a.count from
(   select created_by,count(*)
    from messages
    where messages.created_at>=now()-'24h'::interval
    group by 1 order by 2 desc limit 10 ) a
join users on a.created_by=users.uuid order by 2 desc;

型
如果你真的需要一个函数，你可以将语句 Package 在一个：

create function top_10_chatters_24h()
returns table (name text,count bigint) as $f$
select users.name,a.count from
(   select created_by,count(*)
    from messages
    where messages.created_at>=now()-'24h'::interval
    group by 1 order by 2 desc limit 10 ) a
join users on a.created_by=users.uuid order by 2 desc; 
$f$ language sql;

select * from top_10_chatters_24h();

型
Demo at db<>fiddle