-- table 1 with column list
with user_tab(user_id,user_name,registry_date) as (
select 1,'Bob',to_date('11.02.1921 23:04:44','DD.MM.YYYY HH24:MI:SS') from dual union all
select 2,'Max',to_date('11.02.1922 23:04:44','DD.MM.YYYY HH24:MI:SS') from dual union all
select 3,'Jon',to_date('11.02.1923 23:04:44','DD.MM.YYYY HH24:MI:SS') from dual
)
-- table 2 with column list
,user_post(post_id,user_id,parent_post_id,post_date,message) as (
select 100,1,null,to_date('11.02.1923 23:04:00','DD.MM.YYYY HH24:MI:SS'),'Hi' from dual union all
select 101,2,100,to_date('11.02.1923 23:05:23','DD.MM.YYYY HH24:MI:SS'),'Hi' from dual union all
select 102,3,100,to_date('11.02.1923 23:05:35','DD.MM.YYYY HH24:MI:SS'),'Hi' from dual union all
select 103,1,102,to_date('11.02.1923 23:06:00','DD.MM.YYYY HH24:MI:SS'),'How are you' from dual union all
select 104,2,102,to_date('11.02.1923 23:07:05','DD.MM.YYYY HH24:MI:SS'),'All right!' from dual union all
select 105,1,101,to_date('11.02.1923 23:06:21','DD.MM.YYYY HH24:MI:SS'),'Bye' from dual union all
select 106,1,104,to_date('11.02.1923 23:04:37','DD.MM.YYYY HH24:MI:SS'),'Bye' from dual union all
select 107,2,null,to_date('11.02.1923 23:08:56','DD.MM.YYYY HH24:MI:SS'),'Hmm' from dual
)
-- query sample
select
-- level
-- ,u.user_name
-- ,p.message
rpad(to_char(p.post_date,'YYYY-MM-DD HH24:MI:SS  '),20+level*2,' ')||u.user_name||': '||p.message as txt
from 
 user_post p
 join user_tab  u on ( p.user_id = u.user_id)
where p.post_date >= u.registry_date
start with p.parent_post_id is null 
connect by prior p.post_id=p.parent_post_id 
;

TXT
--------------------------------------------------------------------------------
1923-02-11 23:04:00   Bob: Hi
1923-02-11 23:05:23     Max: Hi
1923-02-11 23:06:21       Bob: Bye
1923-02-11 23:05:35     Jon: Hi
1923-02-11 23:06:00       Bob: How are you
1923-02-11 23:07:05       Max: All right!
1923-02-11 23:04:37         Bob: Bye
1923-02-11 23:08:56   Max: Hmm

8 rows selected.