Hierarchical query with multiple roots

Question

Hi,
I'm trying to write a sql query to find blocking lock in database. The idea is to get the blocker in 1st column, comma separated list of waiters in 2nd column and number of waiters in 3rd column.
As of now i use below query to get a tree structure
WITH lk AS
     (SELECT  blocking_session||' (Inst-'||blocking_instance || ')' blocker,
             SID||' (Inst-'||inst_id || ')' waiter
        FROM gv$session
       WHERE blocking_instance IS NOT NULL AND blocking_session IS NOT NULL)
SELECT     LPAD (' ', 2 * (LEVEL - 1)) || waiter lock_tree
      FROM (SELECT *
              FROM lk
            UNION ALL
            SELECT DISTINCT 'root', blocker
                       FROM lk
                      WHERE blocker NOT IN (SELECT waiter
                                              FROM lk))
CONNECT BY PRIOR waiter = blocker
START WITH blocker = 'root';
Result:-
===========
LOCK_TREE
1966 (Inst-1)
  908 (Inst-1)
  1906 (Inst-1)
  1900 (Inst-1)
981 (Inst-1)
  921 (Inst-1)
  937 (Inst-1)
  962 (Inst-1)
  1889 (Inst-1)
  1904 (Inst-1)
  974 (Inst-1) But what i expect is like below. My below query works when there is only one root blocker, but fails when there are multiple root node.
WITH lk AS
     (SELECT blocking_session || '(Inst-' || blocking_instance || ')' blocker,
             SID || '(Inst-' || inst_id || ')' waiter
        FROM gv$session
       WHERE blocking_instance IS NOT NULL AND blocking_session IS NOT NULL)
SELECT     blocker, SUBSTR (SYS_CONNECT_BY_PATH (waiter, ';'), 2) waiters
      FROM (SELECT   blocker, waiter, ROW_NUMBER () OVER (ORDER BY waiter)
                                                                          val
                FROM lk
            GROUP BY blocker, waiter)
     WHERE CONNECT_BY_ISLEAF = 1
START WITH val = 1
CONNECT BY PRIOR val = val - 1
Result:-
===========
  WAITERS# BLOCKER                                                                                 WAITERS
         3 981(Inst-1)                                                                             1904(Inst-1);921(Inst-1);937(Inst-1)
....lot of duplicates
expected result:-
===========
  WAITERS# BLOCKER                                                                                 WAITERS
         4 981(Inst-1)                                                                             1904(Inst-1);921(Inst-1);937(Inst-1);974(Inst-1)
         3 1966(Inst-1)                                                                             908 (Inst-1);1906 (Inst-1);1900 (Inst-1) can you please help me correct above query or suggest other ways to archive this result.
Thanks in advance
MidhunGT

Answer 1

Hi All,
Thank you all for your support. I never knew these many ways to see blocking lock in database :)
Somehow i was able get the desired result for my specific requirement with below query
sql> WITH lk AS
  2       (SELECT blocking_session || ' (Inst-' || blocking_instance
  3               || ')' blocker,
  4               SID || ' (Inst-' || inst_id || ')' waiter
  5          FROM gv$session
  6         WHERE blocking_instance IS NOT NULL AND blocking_session IS NOT NULL)
  7  SELECT     blocker,
  8             LTRIM
  9                (MAX (SYS_CONNECT_BY_PATH (waiter, ','))KEEP (DENSE_RANK LAST ORDER BY cnt),
10                 ','
11                ) AS waiters,
12             MAX (cnt) waiters#
13        FROM (SELECT blocker, waiter,
14                     ROW_NUMBER () OVER (PARTITION BY blocker ORDER BY waiter)
15                                                                         AS cnt
16                FROM lk)
17    GROUP BY blocker
18  CONNECT BY cnt - 1 = PRIOR cnt AND blocker = PRIOR blocker
19  START WITH cnt = 1;
BLOCKER         WAITERS                                                                            WAITERS#
1946 (Inst-1)   1987 (Inst-1),879 (Inst-1),910 (Inst-1)                                                3
930 (Inst-1)    1919 (Inst-1),1945 (Inst-1),1953 (Inst-1),1983 (Inst-1)                                4please advice, if any scope for improvement
Thanks and Regards,
MidhunGT