ORA-1489 error but only sometimes...when not having distinct or another function working

Question

So when I enter:
SELECT opened_by from(
SELECT  ps."NUMBER",
ps.opened_by,
ps.assignee_name,
ps.closed_by,
listagg(att.type,',') within group(order by att.type) actions,
listagg(att.description_txt,',') within group (order by att.description_txt) descs
from tableps ps
LEFT JOIN activity_txt att ON ps."NUMBER" = att."NUMBER" and att.type in('Assignment','Reassignment')
where open_time between to_date('01-OCT-2012','DD-MON-YYYY') and to_date('31-OCT-2013','DD-MON-YYYY')
and ((ps.opened_by=ps.closed_by) and (ps.closed_by=ps.assignee_name) and (ps.assignee_name=ps.opened_by))
group by ps."NUMBER",ps.opened_by,ps.assignee_name,ps.closed_by
where (actions not like '%Reass%'or actions is null)
and (descs like ('%NONE to '|| opened_by ||'%') or descs is null)
and (descs not like '%,%' or descs is NULL);
All is well in the world...however when I add distinct to the outer select I get the
ORA-01489: result of string concatenation is too long
01489. 00000 -  "result of string concatenation is too long"
*Cause:    String concatenation result is more than the maximum size.
*Action:   Make sure that the result is less than the maximum size.
I don't understand the difference why one would work and one doesn't.         

Answer 1

I little changed your quesry,
Please try, if you are getting same error, then it means you have very  large desciptions in description_txt column.
Pleasech check :
https://forums.oracle.com/thread/2596075
https://forums.oracle.com/message/10601105#10601105
SELECT T."NUMBER",
    T.opened_by,
    T.assignee_name,
    T.closed_by,
    listagg(T.type,',') within GROUP(ORDER BY T.type) actions,
    listagg(T.description_txt,',') within GROUP (ORDER BY T.description_txt) descs
FROM
  (SELECT
    ps."NUMBER",
    ps.opened_by,
    ps.assignee_name,
    ps.closed_by,att.type, att.description_txt
  FROM tableps ps LEFT JOIN activity_txt att ON ps."NUMBER" = att."NUMBER" AND att.type  IN('Assignment','Reassignment')
  WHERE open_time BETWEEN to_date('01-OCT-2012','DD-MON-YYYY') AND to_date('31-OCT-2013','DD-MON-YYYY')
    AND ((ps.opened_by   =ps.closed_by)
    AND (ps.closed_by    =ps.assignee_name)
    AND (ps.assignee_name=ps.opened_by))
     and (att.type NOT LIKE '%Reass%' OR att.type IS NULL)
     AND (att.description_txt LIKE ('%NONE to '|| opened_by ||'%') OR att.description_txt IS NULL)
     AND (att.description_txt NOT LIKE '%,%'OR att.description_txt IS NULL)) T
Group by T."NUMBER",
    T.opened_by,
    T.assignee_name,
    T.closed_by
And carefully read Solomon Yakobson's and Frank Kulash's replies.
Regards
Mahir M. Quluzade