1. Derby
  2. DERBY-4471

Left outer join reassociation rewrite gives wrong result


    • Type: Bug Bug
    • Status: Closed
    • Priority: Major Major
    • Resolution: Fixed
    • Affects Version/s:,,,,,,,,,,,,,,
    • Fix Version/s:,
    • Component/s: SQL
    • Bug behavior facts:
      Wrong query result


      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

      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.

      1. derby-4471-1a.diff
        96 kB
        Dag H. Wanvik
      2. derby-4471-1a.stat
        0.2 kB
        Dag H. Wanvik
      3. derby-4471-1b.diff
        103 kB
        Dag H. Wanvik
      4. derby-4471-1b.stat
        0.2 kB
        Dag H. Wanvik
      5. derby-4471-1c.diff
        2.06 MB
        Dag H. Wanvik
      6. derby-4471-1c.stat
        0.7 kB
        Dag H. Wanvik
      7. derby-4471-1d.diff
        2.06 MB
        Dag H. Wanvik
      8. derby-4471-1d.stat
        0.6 kB
        Dag H. Wanvik
      9. derby-4471-junit-repro.diff
        4 kB
        Dag H. Wanvik
      10. query_plan_derby_4471.pdf
        77 kB
        Nirmal Fernando

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            • Assignee:
              Dag H. Wanvik
              Dag H. Wanvik
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