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Date: Mon, 14 Dec 2009 17:45:18 +0000 (UTC)
From: "Knut Anders Hatlen (JIRA)" <jira@apache.org>
To: derby-dev@db.apache.org
Subject: [jira] Commented: (DERBY-4471) Left outer join reassociation
 rewrite gives wrong result
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    [ https://issues.apache.org/jira/browse/DERBY-4471?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12790258#action_12790258 ] 

Knut Anders Hatlen commented on DERBY-4471:
-------------------------------------------

I haven't studied the details, but at least PostgreSQL agrees with Dag that (a,NULL,NULL,NULL) is the correct result.

> 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
>            Reporter: Dag H. Wanvik
>
> 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.

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