Question about multiple table JOIN syntax

Question

Good morning,
I'm trying to join three tables and getting an error. Here is the query:
with
  p (messageid, parentmessageid, subject) as
    select 2001, null, 'xyz'    from dual union all
    select 2002, 2001, 'ngh'    from dual union all
    select 2003, 2001, 'nghj'   from dual union all
    select 2004, 2001, 'pppp'   from dual union all
    select 2005, null, 'hhhhh'  from dual union all
    select 2006, 2005, 'hhhhj'  from dual
  m (thread, messageid) as
    select 1004, 2001           from dual union all
    select 1005, 2006           from dual
select p2.messageid, p2.parentmessageid, p2.subject
  from (p p2 left outer join m m2
                          on (p2.parentmessageid = m2.messageid))
       join m m3
         on m3.messageid = p2.messageid
;                which gives an "ORA-00918: column ambiguously defined.
I can't figure out why it gives that error. I fail to see the ambiguity.
Thank you for helping,
John.
P.S: In case someone wants to get a good laugh, here is a "solution" that does not have any ambiguity:
with
  parentchild (messageid, parentmessageid, subject) as
    select 2001, null, 'xyz'    from dual union all
    select 2002, 2001, 'ngh'    from dual union all
    select 2003, 2001, 'nghj'   from dual union all
    select 2004, 2001, 'pppp'   from dual union all
    select 2005, null, 'hhhhh'  from dual union all
    select 2006, 2005, 'hhhhj'  from dual
  messagethread (thread, messageid) as
    select 1004, 2001           from dual union all
    select 1005, 2006           from dual
  j1 as
   select p.messageid, p.parentmessageid, p.subject, m.thread
     from parentchild                     p
          left outer join messagethread   m
                       on p.parentmessageid = m.messageid
  j2 as
   select j1.messageid, j1.parentmessageid, j1.subject, j1.thread, m.thread t2
     from j1 left join messagethread m
       on j1.messageid = m.messageid
select j2.messageid, j2.parentmessageid, j2.subject, nvl(j2.thread, t2)
  from j2
order by messageid
;

Answer 1

440bx - 11gR2 wrote:
Hi Sven,
>
I guess the explain plan woudl show that there is some strange query rewrite going on. Where the outer join is added later then needed and therefore the columns do not match anymore as it was in the original query.
>
Is there a "hint" that could be used to test that hypothesis ?
John.I tried NO_QUERY_TRANSFORMATION and MATERIALIZE but the error still was there. So it might not be a problem of rewriting the query.
However I checked the explain plan for a similiar query which is working.
sql> with
  2    p as ( select 2002 messageid, 2001 parentmessageid, 'ngh' subject   from dual)
  3   ,m as ( select 1004 threadx, 2001 messageid2 from dual )
  4   ,m3 as ( select * from m)
  5  select *
  6  from p p2
  7  left join m m2 on p2.parentmessageid = m2.messageid2
  8  join m3 on m3.messageid2 = p2.messageid
  9  ;   
no rows selected
Execution Plan
Plan hash value: 4005120224
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT    |      |     1 |     2 |     0   (0)|          |
|*  1 |  FILTER             |      |       |       |            |          |
|   2 |   NESTED LOOPS OUTER|      |     1 |     2 |     6   (0)| 00:00:01 |
|   3 |    NESTED LOOPS     |      |     1 |       |     4   (0)| 00:00:01 |
|   4 |     FAST DUAL       |      |     1 |       |     2   (0)| 00:00:01 |
|   5 |     FAST DUAL       |      |     1 |       |     2   (0)| 00:00:01 |
|*  6 |    TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter(NULL IS NOT NULL)
   6 - filter(NVL2(ROWID(+),2001,NULL)=2001)
Statistics
          0  recursive calls
          0  db block gets
          0  consistent gets
          0  physical reads
          0  redo size
        357  bytes sent via SQL*Net to client
        231  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed
sql> I can remember some other thread here where a very similiar filter criteria was also mentioned in the context of a "bug".