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Status Not under consideration
Workspace Db2 for z/OS
Created by Guest
Created on Nov 23, 2022

Please do not make stabilizing dynamic query again after a REBIND with APPLCOMPAT a normal behaviour

Hello,

We're DB2z 12 FL508. This is a SAP/R3 application on 2-way data sharing.

We stablized access path of some dynamic SQLs and they worked very well. Note that the packages were at APPLCOMPAT V12R1M500 when we did this.

Lately, the packages were rebound with APPLCOMPAT V12R1M508. We noticed that DB2 stopped using the stabilized access paths thus causing many timeouts on transactions executing those SQLs. We were advised by IBM support that this is a normal behaviour:

"To answer the customer's question about why we don't match when APPLCOMPAT changes, it is normal behavior. We do this because something can change with a different APPLCOMPAT level. The customer should use APPLCOMPAT to ensure there is no behavior change which can cause application problems.

Advancing function level does not affect cache match. It's really about what the package is bound with. If APPLCOMPAT is not specified on the BIND, Db2 for z/OS will pick it up from the zparm. As long as they do not REBIND with a new APPLCOMPAT or change their zparm, then there should be no problem.

So, when a dynamic SQL statement is cached/stabilized, a specific behavior is locked in. Changing APPLCOMPAT can change that behavior, so requires a new cached/stabilized entry."

Why it is considered a normal behaviour for DB2 "not to match" when a rebind to a different applcompat takes place? In my opinion, it makes sense for static but not dynamic SQLs. For us, I see this behaviour a problem. As I mentioned, because DB2 did not use the stablilized access path, we had many timeouts and a severe slowdown of the application then the LPAR.

Is there any plan to introduce a bind parameter or a ZPARM to give us the flexibility to choose whether DB2 should match or not match?


Regards

Needed By Not sure -- Just thought it was cool
  • Admin
    Janet Figone
    Jan 11, 2023

    Dear Mai, Thank you for submitting this enhancement request. Our Db2 for z/OS development team reviewed it and determined it does not align with our product structure. They also provided this feedback for you:

    Since Db2 11, the APPLCOMPAT value is used as a dynamic cached statement matching criteria. This is because Db2 11 introduced the schema SYSIBMADM as part of the default SCHEMA PATH special register which is used to search for user defined functions referenced in a dynamic query. Db2 needs to ensure the same behavior is observed until the application can accept a new behavior change, so APPLCOMPAT is the way to specify that. For example SELECT MYUDF(C1) FROM T1 could choose USER.MYUDF with APPLCOMPAT(V10R1) and SYSIBMADM.MYUDF with APPLCOMPAT(V11R1). The resolved and chosen schema is saved in the cached and stabilized query so when you changed APPLCOMPAT, the saved UDF may not be the right one to execute so we need to go thru full PREPARE.

    APPLCOMPAT continues to be the way to control SQL or application behavior changes and protect the customers from exposure to the unwanted changes until the applications are ready to handle them. As for how to avoid the timeout on full PREPARE, perhaps you can expose the queries gradually after REBIND with changed APPLCOMPAT.

    Consequently, this idea is marked as Not under consideration.

    We do hope that you will continue to submit enhancement requests for our evaluation and consideration to include in the Db2 for z/OS product.

    Sincerely,

    The Db2 for z/OS Team