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Status Future consideration
Workspace Db2 for z/OS
Created by Guest
Created on Nov 17, 2020

UTS PBG, SINGLETON SELECT and Lock Avoidance.

Hello Db2 for Z/OS Development.

Apar PM89016 was created several years ago to implement the lock avoidance for singleton SELECTs with

ISO(CS) and CD(YES) when R-scan is used, or the index that is chosen has

matching predicates on all columns of the index.

Recently a new Apar PH19483 (UI67378) has reverted this behaviour to avoid receiving unexpected SQLCODE +100 when there is INSERT/DELETE/UPDATE activity happening concurrently on the same table.

As can be read on the text of such new Apar PH19483:

"PROBLEM CONCLUSION:

Db2 code is changed to acquire locking for singleton SELECTs meeting the criteria described above. As a result of this change, the user may see increased locking activity and slower query performance for such singleton SELECTs; the locking is mandatory to prevent concurrent operations from causing the incorrect output. If the user desires less locking activity, consider using CURRENTDATA(NO). "

In our case (Bankinter) 80% of our read Accesses are Singleton Selects, resolved with skinny index-to-data (equal predicates on all index columns) access instead of Cursors, so the impact of not "profiting" of lock avoidance feature on those singleton Selects with ISO(CS) and CD(YES) is huge. Furthermore We can't use CURRENTDATA(NO) due to our business processes read with Singleton Select most of the times, but they also read by read-only (or ambiguous) cursors and modify afterwards with Delete or Update statements.

Needed by Date Dec 31, 2020
  • Admin
    Janet Figone
    Dec 17, 2020

    Thanks for bringing this concern. PH19483 resolved intermitted incorrect out with CD(YES) when SELECT and DELETE are executed concurrently. While the change is to honor the semantics of ISO(CS) CD(YES), we understand the performance penalty from the lack of lock avoidance. We will conduct further investigation if there is any solution