IV33576: OPTIMIZER MIGHT FAVOUR HSJOIN OVER AN ORDERED NLJOIN

APAR status

• Closed as fixed if next.

Error description

• The optimizer might favour a hash join (HSJOIN) over a nested
  loop join (NLJOIN) alternative under the following conditions:
  - the join is on two or more columns
  - both tables in the join have an index with leading, non-bound
  key columns that participate in the join
  - one or more leading columns match in order of the join
  columns, but not all
  
  For example, consider the following query
  
  SELECT *
  FROM T1,T2
  WHERE T1.A=T2.A and T1.B=T2.B and T1.C=T2.C and T1.X=1;
  
  where index IX1 is defined on T1(X,A,C,B) and index IX2 is
  defined on T2(A,B,C).  The column T1.X is bound to the constant
  1 as a result of the predicate "T1.X=1" so for the NLJOIN with
  IX1 access on the outer and IX2 access on the inner, the leading
  non-bound columns for both indexes are referenced in the join
  predicate T1.A=T2.A, but the subsequent columns are not ordered
  in join column order.  Under these conditions, the optimizer
  might over estimate the cost of the NLJOIN alternative,
  favouring a possibly worse-performing HSJOIN alternative
  

Local fix

• Create an index on either table, ensuring that the key columns
  are in join order.  Referring to the example in the Error
  Description, this could be achieved by creating an index on
  T2(A,C,B).
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * ALL                                                          *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * See Error Description                                        *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Upgrade to the level of DB2 Version 9.7 containing fix for   *
  * APAR IC90024                                                 *
  ****************************************************************
  

Problem conclusion

Temporary fix

• See Local Fix
  

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

  IC90024

Fix information

Applicable component levels

• R950 PSN

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