Another Sybex vs. STS question

Question

Hi everyone,
I am studying for 1Z0-007. Self test software says that it is illegal to put an order by clause in a view definition; however, page 392 of my sybex study guide says that it is legal, they give an example of it and they go out of their way to point it out...."Notice the ORDER BY clause in the view definition."
Who's right?
Thanks for your help.

Answer 1

Well, Oracle allows you to put ORDER BY in the view definition. But, There might be situations where you do not want to do that.
Consider the case where the ORDER BY column in the view definiton is different and when you actually query the view you want to ORDER BY a different column. The optimizer will perform the SORT of your data twice in this case. See the explain plan for the second query. The SORT is being performed twice. Once for the ORDER BY in the view definition and second time for the ORDER BY in your query from the view.
This is something to watch out for.
SQL> create or replace view emp_view as select * from emp ;
View created.
SQL>
SQL> select * from emp_view order by empno ;
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   TABLE ACCESS (BY INDEX ROWID) OF 'EMP'
   2    1     INDEX (FULL SCAN) OF 'PK_EMP' (UNIQUE)
SQL>
SQL> create or replace view emp_view as select * from emp order by ename ;
View created.
SQL>
SQL> select * from emp_view order by empno ;
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   SORT (ORDER BY)
   2    1     VIEW OF 'EMP_VIEW'
   3    2       SORT (ORDER BY)
   4    3         TABLE ACCESS (FULL) OF 'EMP'
SQL>