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Home -> Community -> Mailing Lists -> Oracle-L -> RE: Re[2]: A Cure for Madness
I agree Niyi -- just to add my two cents:
the SQL language is missing/lacking functionality to implement this
properly.
in relational algebra, "(R WHERE P) WHERE Q" is logically equivalent with "R
WHERE P AND Q" which is obviously also equivalent with the other two
permutations.
obviously, the example that started this thread shows that SQL doesn't have corresponding syntax -- because apparently there are cases where these transformations are not allowed.
see bug 2235753 for an interesting similar case (unfortunately most, if not all, of the bug text is hidden I believe) and I think the only "workaround" in Oracle is to use hints like NO_MERGE or ORDERED_PREDICATES.
I don't think we would like the Oracle optimizer to stop considering subquery flattening, view merging, and predicate pushing in all "possibly suspect" cases, right?
Kind regards,
Lex.
-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org]On Behalf Of Niyi
Sent: Wednesday, September 01, 2004 23:28
To: oracle-l_at_freelists.org; Wolfgang Breitling
Cc: oracle-l_at_freelists.org
Subject: Re: Re[2]: A Cure for Madness
WB> I have said it before and I still content that the original problem is
not
> WB> caused by the optimizer incorrectly reordering the sequence of
> operations
> WB> but by the fact that the to_number function is not well defined over
> the
> WB> domain (varchar2). If that was not the case we would not have this
> debate
> WB> as the end result would be the same no matter the order of operations.
I personally think Oracle's to_number is working as it should, throwing an exception if you try to convert an invalid number, not SILENTLY converting it to null. The fundamental issue here is with predicate ordering determining the result of a query. I don't know the solution to this issue, but I know that to_number is just one of the scenarios that can run into it.
For the simple example below, do you suggest that Oracle not throw division by zero errors too?. I have simulated Jonathan's example, this time with division by values instead of to_number. I am sure other scenarios can be easily deduced.
SQL> select *
2 from v$version;
BANNER
5 rows selected.
SQL> drop table subtest;
Table dropped.
SQL>
SQL> create table subtest(id_type varchar2(1), divisor number(2));
Table created.
SQL> insert into subtest values('P',1);
1 row created.
SQL>
SQL> insert into subtest values('P',2);
1 row created.
SQL> insert into subtest values('Z',0);
1 row created.
SQL>
SQL> insert into subtest values('Z',0);
1 row created.
SQL>
SQL> commit;
Commit complete.
SQL>
SQL> select *
2 from subtest
3 where 1/divisor > 0 and id_type = 'P';
I DIVISOR
- ----------
P 1 P 2
2 rows selected.
SQL> select *
2 from subtest
3 where id_type = 'P' and 1/divisor > 0;
ERROR:
ORA-01476: divisor is equal to zero
no rows selected
SQL>
SQL> select *
2 from (
3 select *
4 from subtest
5 where id_type = 'P')
6 where 1/divisor > 0;
I DIVISOR
- ----------
P 1 P 2
2 rows selected.
SQL>
> WB> And I am sure everyone would be very upset
> WB> if they implemented the rule that a subquery must be evaluated before
> WB> predicates and selection are applied.
>
> Don't forget that Chris is really arguing that the results
> must be "as if" the view had been materialized. Optimization
> is still ok, and I can think of at least two approaches by
> which the subquery can be merged into the main query while
> retaining that "as if" behavior.
>
> WB> I have said it before and I still content that the original problem is
> not
> WB> caused by the optimizer incorrectly reordering the sequence of
> operations
> WB> but by the fact that the to_number function is not well defined over
> the
> WB> domain (varchar2). If that was not the case we would not have this
> debate
> WB> as the end result would be the same no matter the order of operations.
>
> This is actually an interesting line of thought. I like very
> much your my_to_number example. It dovetails with some
> thinking I've been doing lately about null values. Excellent
> example...
>
> Best regards,
>
> Jonathan Gennick --- Brighten the corner where you are
> http://Gennick.com * 906.387.1698 * mailto:jonathan@gennick.com
>
> Join the Oracle-article list and receive one
> article on Oracle technologies per month by
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>
>
> Sunday, August 29, 2004, 12:43:59 PM, Wolfgang Breitling
> (breitliw_at_centrexcc.com) wrote:
> WB> It is probably heresy to contradict the great Chris Date, but I
> disagree
> WB> with this part of his follow-up:
>
> WB> "
> WB> Now, the SQL standard makes it perfectly clear that the result of the
> query
>
> WB> SELECT *
> WB> FROM R1
> WHERE NUM >> 0 ;
>
> WB> is defined as follows:
>
> WB> 1. Evaluate R1.
> WB> 2. Restrict the result of the previous step to just those rows
> satisfying NUM >> 0.
> WB> 3. Project the result of the previous step over all of its
> columns
> WB> (which is effectively a no op, of course).
>
> WB> In other words, the inner subquery must be evaluated before the outer
> WHERE
> WB> and SELECT clauses are executed (hence my unequivocal no to the
> question
> WB> "Is this rewriting on the part of the optimizer valid?"). But there's
> WB> still a little more to be said. "
>
> WB> Maybe that is the SQL standard, I don't know, I don't have the text of
> it,
> WB> but it is certainly not part of the relational theory whose strength
> is the
> WB> fact that the operations selection (where clause), projection (select
> WB> clause) and join can be freely interchanged as long as the rules laid
> out
> WB> by the theory are followed. And none of the relational databases
> implement
> WB> Chris' 1-2-3 order of operation. And I am sure everyone would be very
> upset
> WB> if they implemented the rule that a subquery must be evaluated before
> WB> predicates and selection are applied. Think of the emp table (EMPNO,
> ENAME,
> WB> JOB, MGR, HIREDATE, SAL, COMM, DEPTNO) and create a view that excludes
> the
> WB> sensitive data: emp_vw (select EMPNO, ENAME, JOB, MGR, DEPTNO from
> emp).
> WB> Now, according to Chris' stand, a query like
> WB> select * from emp_vw where empno = 7369 would not be allowed to apply
> the
> WB> predicate to the underlying emp table but would need to evaluate the
> view
> WB> first ( which consists solely of a select , i.e. a temporary rowsource
> WB> consisting of the selected columns from the entire emp table has to be
> WB> built ) before applying the restriction "where empno = 7369". Imagine
> the
> WB> performance.
>
> WB> I have said it before and I still content that the original problem is
> not
> WB> caused by the optimizer incorrectly reordering the sequence of
> operations
> WB> but by the fact that the to_number function is not well defined over
> the
> WB> domain (varchar2). If that was not the case we would not have this
> debate
> WB> as the end result would be the same no matter the order of operations.
> To
> WB> prove it I created a function my_to_number which returns a NULL (i.e.
> WB> undefined) if presented with something that does not represent a
> number:
> WB> create or replace function my_to_number(input in varchar2)
> WB> return number is
> WB> l_num number := NULL;
> WB> begin
> WB> begin
> WB> l_num := to_number(input);
> WB> exception
> WB> when others then null;
> WB> end;
> WB> return l_num;
> WB> end;
>
> SQL>> create table subtest (flag varchar2(1), num varchar2(5));
> WB> Table created.
> SQL>> insert into subtest values('N','123');
> WB> 1 row created.
> SQL>> insert into subtest values('X','123');
> WB> 1 row created.
> SQL>> insert into subtest values('Y','pqr');
> WB> 1 row created.
> SQL>> insert into subtest values('N','456');
> WB> 1 row created.
> SQL>> insert into subtest values('Z','ijk');
> WB> 1 row created.
> SQL>> commit;
> WB> Commit complete.
> SQL>> analyze table subtest compute statistics;
> WB> Table analyzed.
>
> WB> Executing the query as is produces the error:
> SQL>> SELECT SELECT FLAG, NUM
> WB> 2 FROM ( SELECT FLAG, TO_NUMBER ( NUM ) NUM
> WB> 3 FROM SUBTEST
> WB> 4 WHERE FLAG = 'N' )
> WB> 5 WHERE NUM > 0 ;
>
> WB> ERROR:
> WB> ORA-01722: invalid number
>
> WB> Forcing the subquery to be evaluated first gives us the result
> SQL>> SELECT SELECT FLAG, NUM
> WB> 2 FROM ( SELECT rownum, FLAG, TO_NUMBER ( NUM ) NUM
> WB> 3 FROM SUBTEST
> WB> 4 WHERE FLAG = 'N' )
> WB> 5 WHERE NUM > 0 ;
>
> WB> F NUM
> WB> - ----------
> WB> N 123
> WB> N 456
> WB> 2 rows selected.
>
> WB> Using the extended TO_NUMBER function also gives the same result,
> proving
> WB> that the reordering does not affect the final resultset.
> SQL>> SELECT SELECT FLAG, NUM
> WB> 2 FROM ( SELECT FLAG, my_TO_NUMBER ( NUM ) NUM
> WB> 3 FROM SUBTEST
> WB> 4 WHERE FLAG = 'N' )
> WB> 5 WHERE NUM > 0 ;
>
> WB> F NUM
> WB> - ----------
> WB> N 123
> WB> N 456
> WB> 2 rows selected.
>
>
> WB> 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.
>
> WB> Regards
>
> WB> Wolfgang Breitling
> WB> Centrex Consulting Corporation
> WB> www.centrexcc.com
>
> WB> ----------------------------------------------------------------
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