[jira] Commented: (DERBY-4471) Left outer join reassociation rewrite gives wrong result

["Dag H. Wanvik (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/DERBY-4471?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12886924#action_12886924 ] 

Dag H. Wanvik commented on DERBY-4471:
--------------------------------------

A query in DERBY-4436 shows another example of wrong outer join reordering.

https://issues.apache.org/jira/browse/DERBY-4736?focusedCommentId=12886105&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#action_12886105

select * from (t2 left join (t3 left outer join t4 on 1=1) on t2.x = t3.x)  

          loj t2.x=t3.x
          /  \
         /    \
        t2    loj 1=1
              /  \
             /    \
            t3    t4

is rewritten to:

select * from ((t2 left join t3 on t2.x = t3.x) left join t4 on 1=1);

              loj 1=1
              / \
             /   t4
            /
           loj t2.x=t3.x
          /   \
         /     \
       t2     t3

which also gives a wrong result, cf. analysis in DERBY-4736.


> Left outer join reassociation rewrite gives wrong result
> --------------------------------------------------------
>
>                 Key: DERBY-4471
>                 URL: https://issues.apache.org/jira/browse/DERBY-4471
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 10.0.2.0, 10.0.2.1, 10.1.1.0, 10.1.2.1, 10.1.3.1, 10.2.1.6, 10.2.2.0, 10.3.1.4, 10.3.2.1, 10.3.3.0, 10.4.1.3, 10.4.2.0, 10.5.1.1, 10.5.2.0, 10.5.3.0
>            Reporter: Dag H. Wanvik
>         Attachments: derby-4471-junit-repro.diff
>
>
> The following script and output shows the problem:
> > create table r(c1 char(1));
> > create table s(c1 char(1), c2 char(1));
> > create table t(c1 char(1));
> > insert into r values 'a';
> > insert into s values ('b', default);
> > insert into t values ('c');
> > select * from s left outer join t on s.c2=t.c1 or s.c2 is null;
> C1  |C2  |C1  
> --------------
> b   |NULL|c   
> > select * from r left outer join s on r.c1=s.c1;
> C1  |C1  |C2  
> --------------
> a   |NULL|NULL
> > select * from (r left outer join s on r.c1=s.c1) left outer join t on s.c2=t.c1 or s.c2 is null;
> C1  |C1  |C2  |C1  
> -------------------
> a   |NULL|NULL|c   
> > select * from r left outer join (s left outer join t on s.c2=t.c1 or s.c2 is null) on r.c1=s.c1;
> C1  |C1  |C2  |C1  
> -------------------
> a   |NULL|NULL|c   
> The last result is wrong. The correct answer should be:
> C1  |C1  |C2  |C1  
> -------------------
> a   |NULL|NULL|NULL   
> since in the last form, the left table r has the value 'a', which does
> not match any row in result of the compound inner given the join
> predicate ("r.c1=s.c1"), so all nulls should be appended to the 'a'
> from the outer table r.
> This happens because internally the last form is rewritten to the
> second but the last form (left-deep), but this rewrite is not
> justified here unless the join predicate on s rejects null, which the
> present one explicitly does not ("or s.c2 is null"). Cf. for example
> [1], page 52, which describes this transform and its prerequisite
> condition as indentity #7.
> [1] Galindo-Legaria, C. & Rosenthal, A.: "Outerjoin simplification and
> reordering for query optimization", ACM Transactions on Database
> Systems, Vol 22, No 1, March 1997.

-- 
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.