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Status Not under consideration
Workspace Db2 for z/OS
Created by Guest
Created on Mar 29, 2016

RELATED IDEAS

"INDEX ONLY" index scan should be used instead of tablespace scan when it performs better.

We were analyzing the optimization of the following query:

SELECT MAX(A.COL1)
FROM MY_TABLE A
WHERE
A.COL2= ?
WITH UR ;

There is an index that exists for MY_TABLE that has a first column of COL1 and a second column of COL2.

DB2 was using a tablespace scan to deliver the results of this query (even though this index was much smaller than the tablespace). We were able to rewrite the query in the following manner to accomplish a non-matching index scan to deliver the results:

SELECT A.COL1
FROM
MY_TABLE A
WHERE
A.COL2= ?
ORDER BY A.COL1 DESC
FETCH FIRST 1 ROWS ONLY
WITH UR ;

Using an non-matching index scan to deliver the results for this query resulted in significant performance improvement!

We would like to see DB2 make this decision without a manual rewrite.
  • Admin
    Janet Figone
    Reply
    |     Sep 30, 2022

    Dear Larry, Thank you for submitting this enhancement request. The Db2 for z/OS developoment team has re-reviewed it and determined that we can not support this type of rewrite.

    They noted that what you have provided here might be cost model flaw that Db2 did not choose index-only for the first SQL. If you have the recreate docs for us to take a look at, we can try to see why index-only was not chosen for the first SQL.

    In the meantime, because we can not support this type of rewrite, we have dispositioned this idea to not under consideration.

    We appreciate your input to the Db2 for z/OS development team. We also hope that you will continue to submit ideas for improvements as customer feedback is a key component to shaping the future direction of Db2 for z/OS.

    Sincerely,

    Db2 for z/OS Team


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