A Cure for Madness 2004-08-29 - By Wolfgang Breitling
It is probably heresy to contradict the great Chris Date, but I disagree
with this part of his follow-up:
"
Now, the SQL standard makes it perfectly clear that the result of the query
SELECT *
FROM R1
WHERE NUM > 0 ;
is defined as follows:
1. Evaluate R1.
2. Restrict the result of the previous step to just those rows
satisfying NUM > 0.
3. Project the result of the previous step over all of its columns
(which is effectively a no op, of course).
In other words, the inner subquery must be evaluated before the outer WHERE
and SELECT clauses are executed (hence my unequivocal no to the question
"Is this rewriting on the part of the optimizer valid? "). But there 's
still a little more to be said. "
Maybe that is the SQL standard, I don 't know, I don 't have the text of it,
but it is certainly not part of the relational theory whose strength is the
fact that the operations selection (where clause), projection (select
clause) and join can be freely interchanged as long as the rules laid out
by the theory are followed. And none of the relational databases implement
Chris ' 1-2-3 order of operation. And I am sure everyone would be very upset
if they implemented the rule that a subquery must be evaluated before
predicates and selection are applied. Think of the emp table (EMPNO, ENAME,
JOB, MGR, HIREDATE, SAL, COMM, DEPTNO) and create a view that excludes the
sensitive data: emp_vw (select EMPNO, ENAME, JOB, MGR, DEPTNO from emp).
Now, according to Chris ' stand, a query like
select * from emp_vw where empno = 7369 would not be allowed to apply the
predicate to the underlying emp table but would need to evaluate the view
first ( which consists solely of a select , i.e. a temporary rowsource
consisting of the selected columns from the entire emp table has to be
built ) before applying the restriction "where empno = 7369 ". Imagine the
performance.
I have said it before and I still content that the original problem is not
caused by the optimizer incorrectly reordering the sequence of operations
but by the fact that the to_number function is not well defined over the
domain (varchar2). If that was not the case we would not have this debate
as the end result would be the same no matter the order of operations. To
prove it I created a function my_to_number which returns a NULL (i.e.
undefined) if presented with something that does not represent a number:
create or replace function my_to_number(input in varchar2)
return number is
l_num number := NULL;
begin
begin
l_num := to_number(input);
exception
when others then null;
end;
return l_num;
end;
SQL > create table subtest (flag varchar2(1), num varchar2(5));
Table created.
SQL > insert into subtest values( 'N ', '123 ');
1 row created.
SQL > insert into subtest values( 'X ', '123 ');
1 row created.
SQL > insert into subtest values( 'Y ', 'pqr ');
1 row created.
SQL > insert into subtest values( 'N ', '456 ');
1 row created.
SQL > insert into subtest values( 'Z ', 'ijk ');
1 row created.
SQL > commit;
Commit complete.
SQL > analyze table subtest compute statistics;
Table analyzed.
Executing the query as is produces the error:
SQL > SELECT SELECT FLAG, NUM
2 FROM ( SELECT FLAG, TO_NUMBER ( NUM ) NUM
3 FROM SUBTEST
4 WHERE FLAG = 'N ' )
5 WHERE NUM > 0 ;
ERROR:
ORA-01722 (See ORA-01722.ora-code.com): invalid number
Forcing the subquery to be evaluated first gives us the result
SQL > SELECT SELECT FLAG, NUM
2 FROM ( SELECT rownum, FLAG, TO_NUMBER ( NUM ) NUM
3 FROM SUBTEST
4 WHERE FLAG = 'N ' )
5 WHERE NUM > 0 ;
F NUM
- -- ---- --
N 123
N 456
2 rows selected.
Using the extended TO_NUMBER function also gives the same result, proving
that the reordering does not affect the final resultset.
SQL > SELECT SELECT FLAG, NUM
2 FROM ( SELECT FLAG, my_TO_NUMBER ( NUM ) NUM
3 FROM SUBTEST
4 WHERE FLAG = 'N ' )
5 WHERE NUM > 0 ;
F NUM
- -- ---- --
N 123
N 456
2 rows selected.
At 03:19 PM 8/27/2004, you wrote:
>Some of you might have read the article I recently posted on
> "Subquery Madness! ":
>
>http://five.pairlist.net/pipermail/oracle-article/2004/000012.html
>
>Chris Date took an interest in the issue, and he wrote a
>fascinating follow-up, "A Cure for Madness, " which just went
>live today:
>
>http://www.dbdebunk.com/page/page/1351381.htm
>
>Please note that Chris is *not* saying that the subquery
>results need to be materialized, but only that the final
>results need to be "as if " they had been. I can think of at
>least two ways to merge the subquery and yet maintain the
>required "as if " behavior.
>
>Chris 's article is really clear and well-written. I wish I
>could write as well as he does. I think you 'll find his
>article interesting no matter which side of the issue you
>personally fall on.
Regards
Wolfgang Breitling
Centrex Consulting Corporation
www.centrexcc.com
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