PH09814: COST ESTIMATE FOR A QUERY WITH A CORRELATED SUBQUERY IS TOO HIGH

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• In a SQL Query statement, there is local predicate in the parent
  queryblock which covers the column of correlated join column.
  And Db2 overestimates cost to access the subquery.
  .
  Sample Query:
   SELECT * FROM Table1 A WHERE
   keycol_1 = 'literal' AND Decimal_col IN (44,13)
   AND keycol_3 =
   (SELECT MAX(keycol_3)
   FROM Table1 B
   WHERE keycol_1 = 'literal' AND A.Decimal_col = B.Decimal_col)
   ;
  When a query contains a correlated subquery, and there is only
   one table in parent query block, the total cost of subquery is
   overestimated even though correlated join column is covered by
   local predicate in parent query block.
  In this scenario, subquery should get cost reduction.
  .
  Additionally, there are unusual high values for PROCMS and
   PROCSU in DSN_STATEMNT_TABLE, while the query itself runs in
   seconds.
  ---------------------------------------------------------------
  | STMT_TYPE | COST_CATEGORY |     PROCMS     |     PROCSU     |
  ---------------------------------------------------------------
  | SELECT    | A             |       15324080 |      567558656 |
  ---------------------------------------------------------------
  .
  In this particular case, overestimation of costs leads to
   interception of Query by Resource Limit Facility ( RLF ).
  
  Keywords:
   PERFM
   SQLCORRSUBQ
  

Local fix

• n/a
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: Db2 11 and Db2 12 for z/OS users who have a  *
  *                 query that includes a correlated subquery,   *
  *                 and all correlated join columns are covered  *
  *                 by local EQUAL/IN predicates in the parent   *
  *                 query block.                                 *
  ****************************************************************
  * PROBLEM DESCRIPTION: Db2 may overestimate the cost to access *
  *                      a correlated subquery when all the      *
  *                      correlated join columns are covered     *
  *                      by local EQUAL or IN predicates.        *
  ****************************************************************
  * RECOMMENDATION: Apply corrective PTF when available.         *
  ****************************************************************
  When a query includes a correlated subquery, and all the
  correlated join columns are covered by local EQUAL/IN
  predicates in the parent query block, Db2 may overestimate
  the cost to access the subquery.
  
  Example:
  SELECT * FROM T1
  WHERE T1.C1 = ?
  AND T1.C2 IN (?, ?, ?)
  AND T1.C3 = (SELECT MAX(T2.C3) FROM T2
               WHERE T2.C1 = T1.C1
               AND T2.C2 = T1.C2)
  ;
  
  T1.C1 and T1.C2 are correlated join columns, and there are
  local EQUAL/IN predicates that cover these columns in the
  parent query block. Db2 may overestimate the cost to access
  the correlated subquery.
  
  Additional keywords:
  SQLPERFORMANCE SQLACCESSPATH SQLSUBQUERY SQLCORRSUBQ
  

Problem conclusion

• Db2 code has been updated to get a reasonable cost reduction to
  access a correlated subquery in the situation described above.
  

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:

  UI63288 UI63290

Modules/Macros

•    DSNXOCSH DSNXOCSM DSNXOCSN DSNXOPRP DSNXOTS
  

Fix information

• Fixed component name

  DB2 OS/390 & Z/

• Fixed component ID

  5740XYR00

Applicable component levels

• RB10 PSY UI63290

     UP19/06/17 P F906

• RC10 PSY UI63288

     UP19/06/17 P F906

Fix is available

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