To make easier the interpretation of execution plans, in chapter 6 of TOP I defined three types of operations: standalone operations, unrelated-combine operations, and related-combine operations. For combine operations I also added a list of all operations of each type. Since in 11.2 a new related-combine operation is available, I decided to write this short post as addenda to the content of the book.
The new related-combine operation, named “UNION ALL (RECURSIVE WITH)”, is available to support the new recursive subquery factoring clause. Hence, it is used for hierarchical queries. The following query and its execution plan show an example:
SQL> WITH
2 e (xlevel, empno, ename, job, mgr, hiredate, sal, comm, deptno)
3 AS (
4 SELECT 1, empno, ename, job, mgr, hiredate, sal, comm, deptno
5 FROM emp
6 WHERE mgr IS NULL
7 UNION ALL
8 SELECT mgr.xlevel+1, emp.empno, emp.ename, emp.job, emp.mgr, emp.hiredate, emp.sal, emp.comm, emp.deptno
9 FROM emp, e mgr
10 WHERE emp.mgr = mgr.empno
11 )
12 SELECT *
13 FROM e;
-------------------------------------------------------------------------------
| Id | Operation | Name | Starts | A-Rows |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 14 |
| 1 | VIEW | | 1 | 14 |
| 2 | UNION ALL (RECURSIVE WITH) BREADTH FIRST| | 1 | 14 |
|* 3 | TABLE ACCESS FULL | EMP | 1 | 1 |
| 4 | NESTED LOOPS | | 4 | 13 |
| 5 | NESTED LOOPS | | 4 | 13 |
| 6 | RECURSIVE WITH PUMP | | 4 | 14 |
|* 7 | INDEX RANGE SCAN | EMP_MGR | 14 | 13 |
| 8 | TABLE ACCESS BY INDEX ROWID | EMP | 13 | 13 |
-------------------------------------------------------------------------------
3 - filter("MGR" IS NULL)
7 - access("EMP"."MGR"="MGR"."EMPNO")
filter("EMP"."MGR" IS NOT NULL)
Notice that there are actually two operations:
- UNION ALL (RECURSIVE WITH) BREADTH FIRST
- UNION ALL (RECURSIVE WITH) DEPTH FIRST
As their name suggest, the difference is due to the search clause that you can set to either BREADTH FIRST BY or DEPTH FIRST BY.
Reading an execution plan containing the “UNION ALL (RECURSIVE WITH)” operation is the same as reading one containing the “CONNECT BY WITH FILTERING” operation. As a matter of fact, the purpose of both operations is basically the same. Just notice that also the “PUMP” operation used in the execution plan differs. While in the former it is called “RECURSIVE WITH PUMP”, in the latter it is called “CONNECT BY PUMP”. But the difference, for the purpose of reading the execution plan, does not matter.
You find a full description on how to read such an execution plan in this post.
