Sunday, June 30, 2019

Db2 for z/OS - Talking About APPLCOMPAT (Part 1)

The Db2 for z/OS package bind option APPLCOMPAT is not new - it was introduced with Db2 11. Even so, I think that now is a good time to spend some time thinking about APPLCOMPAT, for two reasons: 1) I've encountered some misunderstandings pertaining to APPLCOMPAT, and I'd like to clear those up; and 2) Db2 12 has introduced some changes that relate to APPLCOMPAT, and folks should be aware of those changes.

In this first of a two-part blog entry, I'll cover several aspects of APPLCOMPAT. In the part 2 entry, which I'll post within the next 30 days or so, I'll focus on one particular issue, that being the APPLCOMPAT value used for IBM Data Server Driver / Db2 Connect packages in a Db2 12 for z/OS environment.


What does APPLCOMPAT do?

APPLCOMPAT has a twofold purpose. First, it enables the use of SQL functionality introduced with a Db2 version and/or (in a Db2 12 environment) function level. Consider, for example, the LISTAGG built-in function delivered with Db2 12 function level V12R1M501 (LISTAGG is pretty slick, making it easy to have, as a column in a query result set, a comma-separated list of values). Suppose that an application developer wants to use LISTAGG in a query issued by his program. If the program's package is bound with APPLCOMPAT(V12R1M500) - or any other value below V12R1M501 - then the query that includes LISTAGG will get a SQL error code at execution time (most likely a -4743, ATTEMPT TO USE NEW FUNCTION WHEN THE APPLICATION COMPATIBILITY SETTING IS SET FOR A PREVIOUS LEVEL). The developer will need to change the query so that LISTAGG is not invoked, or the program's package will have to be rebound with an APPLCOMPAT value that is equal to or greater than V12R1M501 (the APPLCOMPAT value in a Db2 12 system can be as high as the system's currently-activated function level).

APPLCOMPAT's other purpose is to provide a shield that protects a program from what the Db2 documentation calls a "SQL incompatibility." I prefer the phrase, "SQL behavioral change," but whether you use that wording or "SQL incompatibility," what you're talking about is a situation in which the same SQL statement, executed with the same data, yields a different result after activation of the functionality provided by a new Db2 version (or a new Db2 12 function level). Here's an example of a SQL incompatibility: while in a Db2 10 system a SQL statement can successfully cast an 8-byte store clock value AS TIMESTAMP, in a Db2 11 new-function mode system that same SQL statement, operating on the same 8-byte store clock value, will get a -180 SQL error code - this because in a Db2 11 NFM environment a store clock value is no longer a valid input for a CAST(... AS TIMESTAMP) operation. Suppose you have a program with a SQL statement that casts a store clock value as a timestamp, and suppose changing that SQL statement (to avoid the -180 error code in a Db2 11 NFM system) is not feasible for some reason. In that case, the program's package could be bound with APPLCOMPAT(V10R1), and as a result the SQL statement will get, in a Db2 11 NFM system (or a Db2 12 system), the same result that it would get in a Db2 10 system, because APPLCOMPAT(V10R1) indicates that behavior for the package's SQL statements is to be what it would be in a Db2 10 system, regardless of the fact that the package is executing in a Db2 11 NFM (or Db2 12) environment (we always document SQL incompatibilities - the change related to casting store a clock value as a timestamp is one of the Db2 11-introduced incompatibilities).


Where does a package's APPLCOMPAT value come from?

As already mentioned, APPLCOMPAT is a package bind option. It is also a ZPARM parameter. That ZPARM has one and only one purpose: it provides the default value of APPLCOMPAT, when needed, for a BIND PACKAGE or REBIND PACKAGE operation. When is the ZPARM-provided default needed? For BIND PACKAGE, it is needed when an APPLCOMPAT value is not specified in the command. For REBIND PACKAGE, it is needed when an APPLCOMPAT value is not specified in the command and "not already there" for the package being rebound. I'll explain what I mean by "not already there." When a package is rebound and an APPLCOMPAT value is not specified in the REBIND PACKAGE command, the package's existing APPLCOMPAT value, if anything, will carry forward through the REBIND operation (so, if a package with an APPLCOMPAT value of V10R1 is rebound and an APPLCOMPAT value is not specified in the REBIND PACKAGE command, the rebound package will retain the V10R1 APPLCOMPAT value). "If anything" implies that the package may not have an explicit APPLCOMPAT value prior to the rebind, and that could indeed be the case (this situation would be indicated by a blank value in the APPLCOMPAT column in the package's row in the SYSIBM.SYSPACKAGE catalog table). When that is so the value used for the REBIND PACKAGE operation will be the value of the APPLCOMPAT parameter in ZPARM (as is the case when BIND PACKAGE is issued sans an APPLCOMPAT specification).

"How about dynamic SQL?" you may ask. "Can't a dynamic SQL-issuing program change the in-effect application compatibility level via the SQL statement SET CURRENT APPLICATION COMPATIBILITY?" Yes, that can be done, but starting with a Db2 12 system for which function level V12R1M500 or higher has been activated, the value specified for the SET CURRENT APPLICATION COMPATIBILITY special register cannot be greater than the APPLCOMPAT value of the program's Db2 package. Any dynamic SQL-issuing program has an associated package, which could be an IBM Data Server Driver / Db2 Connect package. If the package used for a dynamic SQL-issuing program is bound with APPLCOMPAT(V12R1M500) then V12R1M500 will be the initial value of the CURRENT APPLICATION COMPATIBILITY special register when the program executes, and the program can set the value of that special register to something less than V12R1M500 but not to a value greater than V12R1M500.


An optimizer-related misconception

Sometimes people think that APPLCOMPAT does more than it actually does. Case in point: SQL statement optimization. Lots of folks know that it's a good idea to rebind all plans and packages in a Db2 for z/OS system soon after going to a new version of Db2 (one reason: the package code generated by Db2 version n - packages are executable code, you know - is likely to be more CPU-efficient than the code generated for the same package by version n-1). Some of those same people think, however, that to take advantage of new access path choices available with the version n optimizer, APPLCOMPAT(n) - where n would be a value associated with the new Db2 version - has to be specified for the rebind operations. Nope. If you rebind a package with APPLCOMPAT(V11R1) in a Db2 12 system, you will be able to get for that package the performance benefit of access path choices introduced with Db2 12 (assuming that you did not go with the APREUSE option, which tells Db2 to reuse the existing access paths for the package's SQL statements).

Remember: APPLCOMPAT can enable use of new Db2 functionality, and APPLCOMPAT can serve as a shield to insulate a program from a SQL incompatibility introduced by a new version (or new function level) of Db2. APPLCOMPAT does not tell the Db2 optimizer what to do.


Db2 12 and the DDL dimension

In a Db2 11 system, APPLCOMPAT affects SQL DML statements (e.g., SELECT, INSERT, UPDATE and DELETE), but not DDL statements (e.g., CREATE, ALTER). That being the case, in a Db2 11 new-function mode environment a program whose package is bound with V10R1 cannot reference a global variable in a SELECT statement (global variables were introduced with Db2 11), but that same program can create a global variable (because DDL is not affected by APPLCOMPAT in a Db2 11 system).

Starting with Db2 12, APPLCOMPAT affects both DML and DDL statements; so, in a Db2 12 system, a program whose package has an APPLCOMPAT value of V12R1M500 cannot issue an ALTER TABLESPACE statement that includes a KEY LABEL specification (indicating that data in the table is to be encrypted using the key associated with the specified label), because KEY LABEL became an ALTER TABLE option starting with function level V12R1M502, and using that new DDL option would require that the package of the program issuing the ALTER TABLE statement be bound with an APPLCOMPAT value of V12R1M502 or greater.

This DDL dimension of APPLCOMPAT in a Db2 12 system gets particularly interesting starting with function level V12R1M504. With that function level (or higher) activated, a program whose package is bound with APPLCOMPAT(V12R1M504) or higher cannot use static SQL to create a traditional segmented table space or a range-partitioned table space that is not universal partition-by-range (in other words, that program cannot use static SQL to create a table space that is not either a partition-by-growth or a partition-by-range universal table space). Why is this so? Because plenty of Db2-using organizations, cognizant of the advantages of universal table spaces, do not want non-universal table spaces to be even accidentally created. Note also that a program whose package has an APPLCOMPAT value of V12R1M504 or higher cannot use static SQL to create a hash-organized table or a synonym (in the latter case one would create an alias instead).

Why did I repeatedly underline the phrase "cannot use static SQL" in the paragraph above? See the paragraph below.

What if you have a Db2 12 system with function level V12R1M504 or higher activated, and you need to create a traditional segmented table space? You have a couple of options for getting that done. One option would be to use a dynamic SQL-issuing program (an example being the Db2-supplied program called DSNTEP2) whose package has an APPLCOMPAT value of V12R1M504 or higher, and have that program first issue the SQL statement SET CURRENT APPLICATION COMPATIBILITY = 'V12R1M503' and then issue the CREATE for the traditional segmented table space (recall that in a Db2 12 environment in which function level V12R1M500 or higher has been activated, a dynamic SQL-issuing program can set the in-effect application compatibility level to something below the APPLCOMPAT value of its package, but not to something above that APPLCOMPAT value). The other option: create the traditional segmented table space via a program whose package has an APPLCOMPAT value of V12R1M503 or lower.

That's all for now. Tune in in about a month to see part 2 of this blog entry and get some guidance on specifying APPLCOMPAT for the IBM Data Server Driver / Db2 Connect packages in a Db2 12 system.

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