Is it OK to join to a view twice in the same select?

Question

For instance
In this case RPT_PRIMARY_PATRON_DETAIL is a view.
SELECT
PATRON_ORDER.FINANCIAL_PATRON_ACCOUNT_ID,
FIN_PATRON_DETAIL.PATRON_ACCOUNT_NAME,
PATRON_ORDER.ATTENDING_PATRON_ACCOUNT_ID,
SO_PATRON_DETAIL.PATRON_ACCOUNT_NAME
FROM
RPT_COMMON_ORDER_LINE_ITEM
INNER JOIN PATRON_ORDER ON
PATRON_ORDER.ORDER_ID = RPT_COMMON_ORDER_LINE_ITEM.ORDER_ID
LEFT OUTER JOIN RPT_PRIMARY_PATRON_DETAIL FIN_PATRON_DETAIL ON
FIN_PATRON_DETAIL.PATRON_ACCOUNT_ID = PATRON_ORDER.FINANCIAL_PATRON_ACCOUNT_ID
LEFT OUTER JOIN RPT_PRIMARY_PATRON_DETAIL SO_PATRON_DETAIL ON
SO_PATRON_DETAIL.PATRON_ACCOUNT_ID = PATRON_ORDER.ATTENDING_PATRON_ACCOUNT_ID
WHERE
RPT_COMMON_ORDER_LINE_ITEM.REPORT_PROCESS_ID = P_REPORT_PROCESS_ID

Answer 1

riedelme wrote:
As BluShadow and Mohommed stated there should be no problem with functionality if you do it correctly. But joining views does not usually work too well performance-wise as the temp tables resulting from view use don't have indexes for effecient nested loops joins. Performance on view joins can be bad, as can using views of views.Are you sure? Views can use the indexes of the base tables they are based on as a view is just a SQL query definition that is embedded into the overall select statement at execution time...
Noddy example, but the first query using view and the second using direct base table access...
SQL> ed
Wrote file afiedt.buf
  1  select myemps.empno, myemps.ename, myemps2.deptno, myemps3.empno
  2  from myemps, myemps myemps2, myemps myemps3
  3  where myemps.empno = myemps2.empno
  4* and   myemps.empno = myemps3.empno
SQL> /
     EMPNO ENAME          DEPTNO      EMPNO
      7782 CLARK              10       7782
      7839 KING               10       7839
      7934 MILLER             10       7934
Execution Plan
Plan hash value: 1213642587
| Id  | Operation                     | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT              |              |     3 |    84 |     3   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                 |              |     3 |    84 |     3   (0)| 00:00:01 |
|   2 |   NESTED LOOPS                |              |     3 |    63 |     3   (0)| 00:00:01 |
|   3 |    TABLE ACCESS BY INDEX ROWID| EMP          |     3 |    42 |     2   (0)| 00:00:01 |
|*  4 |     INDEX RANGE SCAN          | DEPT_IDX     |     3 |       |     1   (0)| 00:00:01 |
|*  5 |    TABLE ACCESS BY INDEX ROWID| EMP          |     1 |     7 |     1   (0)| 00:00:01 |
|*  6 |     INDEX UNIQUE SCAN         | SYS_C0054243 |     1 |       |     0   (0)| 00:00:01 |
|*  7 |   TABLE ACCESS BY INDEX ROWID | EMP          |     1 |     7 |     0   (0)| 00:00:01 |
|*  8 |    INDEX UNIQUE SCAN          | SYS_C0054243 |     1 |       |     0   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   4 - access("DEPTNO"=10)
   5 - filter("DEPTNO"=10)
   6 - access("EMPNO"="EMPNO")
   7 - filter("DEPTNO"=10)
   8 - access("EMPNO"="EMPNO")
SQL> ed
Wrote file afiedt.buf
  1  select myemps.empno, myemps.ename, myemps2.deptno, myemps3.empno
  2  from emp myemps, emp myemps2, emp myemps3
  3  where myemps.empno = myemps2.empno
  4  and   myemps.empno = myemps3.empno
  5  and   myemps.deptno = 10
  6  and   myemps2.deptno = 10
  7* and   myemps3.deptno = 10
SQL> /
     EMPNO ENAME          DEPTNO      EMPNO
      7782 CLARK              10       7782
      7839 KING               10       7839
      7934 MILLER             10       7934
Execution Plan
Plan hash value: 1213642587
| Id  | Operation                     | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT              |              |     3 |    84 |     3   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                 |              |     3 |    84 |     3   (0)| 00:00:01 |
|   2 |   NESTED LOOPS                |              |     3 |    63 |     3   (0)| 00:00:01 |
|   3 |    TABLE ACCESS BY INDEX ROWID| EMP          |     3 |    42 |     2   (0)| 00:00:01 |
|*  4 |     INDEX RANGE SCAN          | DEPT_IDX     |     3 |       |     1   (0)| 00:00:01 |
|*  5 |    TABLE ACCESS BY INDEX ROWID| EMP          |     1 |     7 |     1   (0)| 00:00:01 |
|*  6 |     INDEX UNIQUE SCAN         | SYS_C0054243 |     1 |       |     0   (0)| 00:00:01 |
|*  7 |   TABLE ACCESS BY INDEX ROWID | EMP          |     1 |     7 |     0   (0)| 00:00:01 |
|*  8 |    INDEX UNIQUE SCAN          | SYS_C0054243 |     1 |       |     0   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   4 - access("MYEMPS"."DEPTNO"=10)
   5 - filter("MYEMPS2"."DEPTNO"=10)
   6 - access("MYEMPS"."EMPNO"="MYEMPS2"."EMPNO")
   7 - filter("MYEMPS3"."DEPTNO"=10)
   8 - access("MYEMPS"."EMPNO"="MYEMPS3"."EMPNO")
SQL>No difference.