PK60758: CAP CARDINALITY USED FOR COSTING WITH "FETCH FIRST N ROWS"

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• The cardinality used for costing of nested table
  expressions with "FETCH FIRST n ROWS" does not take into
  account the reduced rows.
  .
  SQLACCESSPATH SQLPERFORMANCE SQLFFNR
  

Local fix

• N/A
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: All DB2 for z/OS users with queries          *
  *                 containing "FETCH FIRST n ROWS ONLY".        *
  ****************************************************************
  * PROBLEM DESCRIPTION: The estimated number of rows for        *
  *                      SELECT statements with the FETCH FIRST  *
  *                      n ROWS ONLY clause is not capped during *
  *                      the process of access path selection    *
  *                      for the query at bind time.             *
  *                      For example,                            *
  *                                                              *
  *                      SELECT cols                             *
  *                      FROM T1,                                *
  *                           (SELECT cols FROM T2               *
  *                            FETCH FIRST 100 ROWS ONLY) AS X   *
  *                      WHERE T1.C1 = X.C1                      *
  *                                                              *
  *                      ONECOMPROWS for table 'X' in query      *
  *                      block 1 should be 100 rows.             *
  *                                                              *
  *                      This may cause a wide variety of        *
  *                      problems in the plan selection,         *
  *                      including slow query performance.       *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  The estimated number of rows for SELECT statements with the
  FETCH FIRST n ROWS ONLY clause is not capped during the process
  of access path selection for the query during bind time.
  For example,
  
  SELECT cols
  FROM T1, (SELECT cols FROM T2
            FETCH FIRST 100 ROWS ONLY) AS X
  WHERE T1.C1 = X.C1
  
  ONECOMPROWS for table 'X' in query block 1 should be 100 rows.
  
  Additional Keywords:
  SQLACCESSPATH SQLEXPLAIN SQLFFNR DSN_DETCOST_TABLE
  

Problem conclusion

• The bind time code for selecting an access path is changed to
  cap the estimated number of rows coming out of the subquery
  block at n rows if "FETCH FIRST n ROWS ONLY" is specified
  on this subquery block.  This will give DB2 a better chance
  of selecting a better access path for the given query.
  

Temporary fix

Comments

APAR Information

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

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

  UK34882

Modules/Macros

• DSNXOGP
  

Fix information

• Fixed component name

  DB2 OS/390 & Z/

• Fixed component ID

  5740XYR00

Applicable component levels

• R910 PSY UK34882

     UP08/04/08 P F804

Fix is available

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