DISTINCT on object type collection not working with GROUP BY

Question

Hello,
I have an object type with collection defined as:
create or replace type individu_ot as object (
   numero_dossier          number(10),
   code_utilisateur        varchar2(8 char),
   nom                     varchar2(25 char),
   prenom                  varchar2(25 char),
   map member function individu_map return number
create or replace type body individu_ot is
   map member function individu_map return number
   is
   begin
      return SELF.numero_dossier;
   end individu_map;
end;
create or replace type individu_ntt is table of individu_ot
/When I use it in simple SQL without any aggregation, the distinct keyword works well and returns me the distinct entry of my object type in the collection:
SQL> select cast(collect(distinct individu_ot(indivmc.numero_dossier, indivmc.idul, indivmc.nom, indivmc.prenom)) as individu_ntt) as distinct_list
from   site_section_cours    sisc
          inner join enseignant_section_mc   ensemc
          on sisc.code_session = ensemc.code_session and
             sisc.numero_reference_section_cours = ensemc.numero_reference_section_cours
          inner join individu_mc indivmc
          on ensemc.numero_dossier_pidm = indivmc.numero_dossier
where  sisc.seq_site_cours = 6
DISTINCT_LIST(NUMERO_DOSSIER,CODE_UTILISATEUR,NOM,PRENOM)                                                                                                                                                                                            
INDIVIDU_NTT(INDIVIDU_OT(15,PROF5,Amidala,Padmé))                                                                                                                                                                                                    
1 row selected.However in SQL with broader selection with group by, the distinct isn't working anymore.
SQL> select *
from   (
         select sisc.seq_site_cours,
                cast(collect(distinct individu_ot(indivmc.numero_dossier, indivmc.idul, indivmc.nom, indivmc.prenom)) as individu_ntt) as distinct_list
         from   site_section_cours      sisc
                   inner join enseignant_section_mc   ensemc
                   on sisc.code_session = ensemc.code_session and
                      sisc.numero_reference_section_cours = ensemc.numero_reference_section_cours
                   inner join individu_mc indivmc
                   on ensemc.numero_dossier_pidm = indivmc.numero_dossier
         group by sisc.seq_site_cours
where seq_site_cours = 6
SEQ_SITE_COURS DISTINCT_LIST(NUMERO_DOSSIER,CODE_UTILISATEUR,NOM,PRENOM)                                                                                                                                                                                            
             6 INDIVIDU_NTT(INDIVIDU_OT(15,PROF5,Amidala,Padmé),INDIVIDU_OT(15,PROF5,Amidala,Padmé),INDIVIDU_OT(15,PROF5,Amidala,Padmé),INDIVIDU_OT(15,PROF5,Amidala,Padmé),INDIVIDU_OT(15,PROF5,Amidala,Padmé))                                                    
1 row selected.there is case where I need to return more than one collections, with distinct entries in it.
Is there something I am missing?
Thanks
Bruno

Answer 1

Not a bug, rather an undocumented feature.
Here are some alternatives you might want to test :
1) Using the SET operator to eliminate duplicates :
SELECT sisc.seq_site_cours,
       set(
         cast(
           collect(individu_ot(indivmc.numero_dossier, indivmc.idul, indivmc.nom, indivmc.prenom))
           as individu_ntt
       ) as distinct_list
FROM site_section_cours sisc
     INNER JOIN enseignant_section_mc ensemc
             ON sisc.code_session = ensemc.code_session
             AND sisc.numero_reference_section_cours = ensemc.numero_reference_section_cours
     INNER JOIN individu_mc indivmc
             ON ensemc.numero_dossier_pidm = indivmc.numero_dossier
GROUP BY sisc.seq_site_cours
;2) Using MULTISET with a subquery
SELECT sisc.seq_site_cours,
       CAST(
         MULTISET(
           SELECT distinct
                  indivmc.numero_dossier, indivmc.idul, indivmc.nom, indivmc.prenom
           FROM enseignant_section_mc ensemc
                INNER JOIN individu_mc indivmc
                        ON ensemc.numero_dossier_pidm = indivmc.numero_dossier
           WHERE sisc.code_session = ensemc.code_session
           AND sisc.numero_reference_section_cours = ensemc.numero_reference_section_cours
         AS individu_ntt
       ) AS distinct_list
FROM site_section_cours sisc
;