Does an inline view execute before the rest of the select?

Question

Hi,
The O'Reilly book Mastering Oracle Sql defines an inline view simply as a
subquery in a select FROM clause. This conforms to the definitions in
the Oracle 9i and 10g docs.
But unlike the Oracle docs, the O'Reilly book says the inline view is,
like a WITH clause, executed before the rest of the query. I can't see
how this can be done unless the inline view makes no reference to
other tables joined in the FROM clause.
Is there a species of inline view in 9i/10g that IS restricted as above -
no refs to other tables in the FROM clause - and what is it called?
Thanks for any help.
Ken Quirici

Answer 1

As explained above, Oracle can choose to merge the query if feasible to do so. Look at the two queries and their plans below. The plan in second query changes to creating a temporary VIEW since in that case, Oracle is unable to perform the merge:
SQL> select *
  2          from scott.emp e,
  3                  (select * from scott.dept d where d.deptno = 10) dd
  4  where
  5          e.deptno = dd.deptno
  6  /
     EMPNO ENAME      JOB              MGR HIREDATE           SAL       COMM     DEPTNO     DEPTNO DNAME          LOC
      7782 CLARK      MANAGER         7839 09-JUN-1981       2450                    10         10 ACCOUNTING     NEW YORK
      7839 KING       PRESIDENT            17-NOV-1981       5000                    10         10 ACCOUNTING     NEW YORK
      7934 MILLER     CLERK           7782 23-JAN-1982       1300                    10         10 ACCOUNTING     NEW YORK
3 rows selected.
Execution Plan
Plan hash value: 3570059291
| Id  | Operation                    | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT             |                |     3 |   171 |     2   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |                |     3 |   171 |     2   (0)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT           |     1 |    20 |     1   (0)| 00:00:01 |
|*  3 |    INDEX UNIQUE SCAN         | PK_DEPT        |     1 |       |     0   (0)| 00:00:01 |
|   4 |   TABLE ACCESS BY INDEX ROWID| EMP            |     3 |   111 |     1   (0)| 00:00:01 |
|*  5 |    INDEX RANGE SCAN          | IDX_EMP_DEPTNO |     3 |       |     0   (0)| 00:00:01 |
SQL> select *
  2          from scott.emp e,
  3                  (select * from scott.dept d where d.deptno = 10 and rownum > 0) dd
  4  where
  5          e.deptno = dd.deptno
  6  /
     EMPNO ENAME      JOB              MGR HIREDATE           SAL       COMM     DEPTNO     DEPTNO DNAME          LOC
      7782 CLARK      MANAGER         7839 09-JUN-1981       2450                    10         10 ACCOUNTING     NEW YORK
      7839 KING       PRESIDENT            17-NOV-1981       5000                    10         10 ACCOUNTING     NEW YORK
      7934 MILLER     CLERK           7782 23-JAN-1982       1300                    10         10 ACCOUNTING     NEW YORK
3 rows selected.
Execution Plan
Plan hash value: 1670713873
| Id  | Operation                       | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT                |                |     5 |   280 |     2   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                   |                |     5 |   280 |     2   (0)| 00:00:01 |
|   2 |   VIEW                          |                |     1 |    19 |     1   (0)| 00:00:01 |
|   3 |    COUNT                        |                |       |       |            |          |
|*  4 |     FILTER                      |                |       |       |            |          |
|   5 |      TABLE ACCESS BY INDEX ROWID| DEPT           |     1 |    20 |     1   (0)| 00:00:01 |
|*  6 |       INDEX UNIQUE SCAN         | PK_DEPT        |     1 |       |     0   (0)| 00:00:01 |
|   7 |   TABLE ACCESS BY INDEX ROWID   | EMP            |     5 |   185 |     1   (0)| 00:00:01 |
|*  8 |    INDEX RANGE SCAN             | IDX_EMP_DEPTNO |     5 |       |     0   (0)| 00:00:01 |
SQL> disconnect
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL>
can it, in it's WHERE clause, reference other tables in the FROM clause?No. it cannot. but as said above, tha may not stop Oracle from merging the queries.
SQL> select *
  2          from scott.emp e,
  3                  (select * from scott.dept d where d.deptno = 10 and e.sal > 1000) dd
  4  where
  5          e.deptno = dd.deptno
  6  /
                (select * from scott.dept d where d.deptno = 10 and e.sal > 1000) dd
ERROR at line 3:
ORA-00904: "E"."SAL": invalid identifier
SQL>