IT31995: AFTER UPGRADE FROM V105, DB2 MAY BE UNABLE TO MATCH ELIGIBLE XMLINDEXES FOR XQUERY, LEADING TO POOR QUERY PERFORMANCE.

Db2 Version 11.1 Mod 4 Fix Pack 6 for Linux, UNIX, and Windows
Db2 Version 11.1 Mod 4 Fix Pack 6 for Linux, UNIX, and Windows

APAR status

• Closed as program error.

Error description

• If the below conditions are met, Db2 is not able to match
  eligible XML indexes which may lead to poor query performance.
  Indexes will not be able to be forced via guideline.
  
  1. coexistance of index on XML column with expression based
  index on another non-XML column for the same table.
  
  2. use of XMLTABLE function on a table with as described in 1 of
  the defined indexes
  
  3. the XMLTABLE call and XML index definition refers the to the
  same XPath or subpath:
  
  e.g.
  using xmlpattern '/a/b/c'
  XMLTABLE('$XMLCOL/a' COLUMNS J varchar(255) path 'b/c')
  
  
  simplified repro:
  
  drop table TXML;
  
  create table TXML (
    id integer not null generated always as identity,
    x varchar(255),
    xmlcol xml
  ) organize by row;
  
  alter table TXML add primary key (id);
  
  create index ix1 on TXML(xmlcol)
    generate key using xmlpattern '/a/b/c'
     as sql varchar (60 octets) ignore invalid values;
  
  create index ix2 on TXML (lower(x));
  
  explain plan for
  SELECT
    xt.J,
    ID
  FROM
    TXML,
    XMLTABLE('$XMLCOL/a' COLUMNS J varchar(255) path 'b/c') as xt
  WHERE
    ((xt.J = 'XYZ')) ;
  

Local fix

• query can possibly be rewritten to avoid listed conditions
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users with XML indexes that meet APAR criteria           *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * AFTER UPGRADE FROM V105, DB2 MAY BE UNABLE TO MATCH ELIGIBLE *
  * XML INDEXES FOR XQUERY, LEADING TO POOR QUERY PERFORMANCE.   *
  *                                                              *
  * If the below conditions are met, Db2 is not able to match    *
  * eligible XML indexes which may lead to poor query            *
  * performance.                                                 *
  * Indexes will not be able to be forced via guideline.         *
  *                                                              *
  * 1. coexistance of index on XML column with expression based  *
  * index on another non-XML column for the same table.          *
  * 2. use of XMLTABLE function on a table with as described in  *
  * 1 of                                                         *
  * the defined indexes                                          *
  * 3. the XMLTABLE call and XML index definition refers the to  *
  * the                                                          *
  * same XPath or subpath:                                       *
  * e.g.                                                         *
  * using xmlpattern '/a/b/c'                                    *
  * XMLTABLE('$XMLCOL/a' COLUMNS J varchar(255) path 'b/c')      *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Apply fix pack containing code fix                           *
  ****************************************************************
  

Problem conclusion

• This fix will correct the issue where Query Rewrite does not
  perform index matching in the outlined conditions.
  

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:

Fix information

• Fixed component name

  DB2 FOR LUW

• Fixed component ID

  DB2FORLUW

Applicable component levels

• RB10 PSN

     UP

• RB10 PSY

     UP

• RB50 PSN

     UP

• RB50 PSY

     UP