Question

I hava data in an Oracle table that is organized as a graph that can contain cycles (see example).

     CREATE TABLE T (parent INTEGER, child INTEGER)
               AS select 1 parent, 2 child from dual
        union all select 1 parent, 8 child from dual
        union all select 2 parent, 3 child from dual
        union all select 2 parent, 4 child from dual
        union all select 2 parent, 8 child from dual
        union all select 3 parent, 4 child from dual
        union all select 3 parent, 6 child from dual
        union all select 4 parent, 5 child from dual
        union all select 5 parent, 8 child from dual
        union all select 6 parent, 5 child from dual
        union all select 7 parent, 3 child from dual
        union all select 7 parent, 5 child from dual
        union all select 8 parent, 6 child from dual

My goal is to get all nodes that are descendants (children, children of children, etc.) of node X. Let’s say 2. My expected result is then: 3, 4, 5, 6, 8.

I know that I can design a query like this:

SELECT child, sys_connect_by_path(child,'/')
   FROM T
  START WITH parent = 2
CONNECT BY NOCYCLE PRIOR child = PARENT;

The problem with such a query is that it will go through all possible paths until they cycle, and there are way too many of them in my actual data. The result consists of many duplicates – Here it is:

child | sys_connect_by_path (for information)
3     | /3
4     | /3/4
5     | /3/4/5
8     | /3/4/5/8
6     | /3/4/5/8/6
6     | /3/6
5     | /3/6/5
8     | /3/6/5/8
4     | /4
5     | /4/5
8     | /4/5/8
6     | /4/5/8/6
8     | /8
6     | /8/6
5     | /8/6/5

My actual data is much more complex. the cost of execution of such a query is so huge that my TEMP tablespace, which was autoextendable, reached 10Gb (originally 500 Mb) and my database actually broke because of disk full.

I tried to design the query like this (recursive WITH clause) :

WITH descendants(node) AS
( SELECT 2 node FROM dual
  UNION ALL
  (
  SELECT child
    FROM T
   INNER JOIN descendants D
      ON T.parent = D.node
   MINUS SELECT node FROM descendants
  )
)
SELECT * FROM descendants

The problem that I encounter is:

• with Oracle 10g, this is not implemented (ORA-32033: unsupported column aliasing, and some customers use Oracle 9 or 10),
• with Oracle 11g, I get ORA-32041: UNION ALL operation in recursive WITH clause must have only two branches. If I remove the MINUS clause I will get cycles (ORA-32044: cycle detected while executing recursive WITH query).

How would you query my original data to get those nodes 3, 4, 5, 6, 8 efficiently? PL/SQL solutions are also welcome.

Thank you.

Answer 1

What is your expected maximum depth to reach any child node?

If it’s relatively small, you could loop down, while checking for nodes you have already visited, in a manner something like this…

(Note, I’m not an Oracle expert so this is closer to pseudo code with a little real SQL mixed in)

CREATE TABLE myMap (parent INT, child INT);

INSERT INTO myTable SELECT NULL, 2 FROM DUAL;

WHILE (SQL%ROWCOUNT > 0)
LOOP

  INSERT INTO
    myMap
  SELECT DISTINCT
    dataMap.parent,
    dataMap.child
  FROM
    myMap
  INNER JOIN
    dataMap
      ON myMap.child = dataMap.parent
  WHERE
    NOT EXISTS (SELECT * FROM myMap WHERE parent = dataMap.parent)

END LOOP;

Depending on performance, you may also want a depth field in myMap; optimising the join so as to only join on the most recent nodes. This would imply two indexes; one for the JOIN (depth) and one for the NOT EXISTS (parent).

EDIT

Added the DISTINCT key word, to avoid the following case…
– Node 2 maps to 3 and 4
– Nodes 3 and 4 both map to node 5
– All children of node 5 would now be processed twice

GROUP BY, or many other options, can be used to cater for this instead of DISTINCT. It’s just that the NOT EXISTS on it’s own is not sufficient.