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Status Not under consideration
Workspace Db2 for z/OS
Created by Guest
Created on Jul 14, 2020

RELATED IDEAS

Improve performance of except statement

Hi,

For the first time I had a use for the except statement. It is quite simple:

SELECT DISTINCT LOG_PGM

FROM BDSTB030_BDSLOG

EXCEPT

SELECT LOG_PGM

FROM BDSTB031_BDSLOG_SLET

I want all LOG_PGM from the first table, that is not in the second table. LOG_PGM is unique in the second table, not in the first.

There is 250 rows in the second table, and 370M rows in the first, spread over 380 LOG_PGM

This returns the following Processor Milliseconds: 1997980 and Processor Service Units: 73999264

I did not understand why this was happening.
In the first table there is an index on LOG_PGM, so doing the select distinct LOG_PGM should be fast.

I tried just that part of the SQL, and it is FAST, with the following processor numbers: 2 and 73.

So I did the following:

SELECT LOG_PGM

FROM (

SELECT DISTINCT LOG_PGM

FROM BDSTB030_BDSLOG )

EXCEPT

SELECT LOG_PGM

FROM BDSTB031_BDSLOG_SLET

This causes DB2 to materialize the first part of the except before doing the except

this gives processor numbers: 3 and 84, a great improvement to the first SQL.

I think DB2 should be able to determine this approach by itself, and therefore give better performance to all SQL of this type.

Thanks

Regards,

Rune Christensen

Needed by Date Oct 20, 2020
  • Admin
    Janet Figone
    Reply
    |     Dec 10, 2020

    Thank you for submitting this Db2 for z/OS enhancement request.

    After giving the request a comprehensive review, we have determined that we cannot include it as a candidate in an upcoming product deliverable.

    While Db2 agrees that there are opportunities to improve the performance of EXCEPT, the low customer usage of this feature (and low number of votes for this requirement) means that Db2 will not be able to prioritize this enhancement within the next release.

    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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