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https://issues.apache.org/jira/browse/DERBY-4471?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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C.S. Nirmal J. Fernando updated DERBY-4471:
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Attachment: query_plan_derby_4471.pdf
Hi,
I sincerely hope that this PDF (created using
screen shots of the query plan imported
by PlanExporter tool (Derby-4587))
helps you to get an idea of what's happening.
If you like any other data in Xplain tables appear in the
plan, please suggest.
Thanks.
> 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, query_plan_derby_4471.pdf
>
>
> 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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