The Path to an Executable DB2 Program

Here is another Q+A exchange that I thought might be useful to share with everyone here on the blog:

QUESTION: Could you please explain the difference between a package and a plan, the process of precompilation, compilation and running a COBOL DB2 program as well as a COBOL CICS DB2 program?

ANSWER: Well, I'll hit the highlights in response to your question but if you really want an in-depth answer then you should read the DB2 manuals (or a book like DB2 Developer's Guide).

What is a plan?

A plan is an executable module containing the access path logic produced by the DB2 optimizer. It can be composed of one or more DBRMs and packages. Before a DB2 for z/OS program (with static SQL) can be run, it must have a plan associated with it.

Plans are created by the BIND command. The plan is stored in the DB2 directory and accessed when its program is run. Information about the plan is stored in the DB2 catalog.

What is a package?

A package is a single, bound DBRM with optimized access paths. By using packages, the table access logic is "packaged" at a lower level of granularity than a plan -- at the package (or program) level.

To execute a package, you first must include it in the package list of a plan (usually, there are some exceptions, such as for triggers and user-defined functions). In general, packages are not directly executed, they are only indirectly executed when the plan in which they are contained executes -- as discussed previously, UDFs and triggers are exceptions to this rule. A plan can consist of one or more DBRMs, one or more packages or, a combination of packages and DBRMs.
What is program preparation?

A DB2 application program must go through a process known as program preparation before it can run successfully. Program preparation requires a series of code preprocessors that when enacted in the proper sequence, create an executable load module and a DB2 application plan. The combination of the executable load module and the application plan is required before any DB2 program can be run, whether batch or online. CICS programs require an additional preprocessing step.

The first step is precompilation. DB2 programs must be parsed and modified before normal compilation. The DB2 precompiler performs this task. At a high level, the precompiler basically searches for SQL, strips it out of the program and puts it into a DBRM. The output of precompilation is a DBRM with SQL and a modified source program.

The DBRM is bound using the BIND command, which is a type of compiler for SQL statements. In general, BIND reads SQL statements from DBRMs and produces a mechanism to access data as directed by the SQL statements being bound.

The modified source is compiled using the language compiler of your choice. The compiled source then is link-edited to an executable load module. The appropriate DB2 host language interface module also must be included by the link edit step. This interface module is based on the environment (TSO, CICS, or IMS/TM) in which the program will execute.

At the end of this series of steps, if all goes well, you will have an executable DB2 program.