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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