Db2 for z/OS and Managing Database Changes - Part 2

In part 1 of our multi-part series on Db2 for z/OS database change management, we provided an overview of the three types of database change that can be undertaken. In today's post, we are going to examine the first type of change -- the simple database change -- in a little more depth.

Simple database changes are the easiest to implement. A simple database change, typically implemented using the ALTER statement, can be executed immediately upon request. The change is made immediately but may require additional actions to fully implement. For example, if you add a nullable column at the end of a table using ALTER TABLE ADD COLUMN the change is made immediately. For all intents and purposes, the addition is complete. However, under the covers, Db2 has not expanded the storage for each row to include space for the column. This happens as the column is accessed and used, or when the table space is reorganized. Applications can use and access the new column without knowing this, however, so the change is immediate; housekeeping to implement the change entirely may occur over time.

Additional examples of simple, immediate changes are most CREATE and DROP statements; altering STOGROUPs; altering most default parameters for databases, table spaces, indexes, and STOGROUPs; renaming tables (packages are invalidated but privileges and indexes are maintained)[1]; renaming indexes; adding a column at the end of a table[2]; changing the data type[3], precision scale of length of a column[4]; identity column parameters, adding and dropping versioning to a temporal table, adding and dropping constraints[5]; activating and deactivating row access control; adding, dropping, and exchanging clone tables; altering, dropping, and refreshing materialized query tables[6]; creating, dropping, and renaming global temporary tables; altering most aspects of user-defined functions and stored procedures; and changing or dropping labels on tables, aliases, and columns. 

Additionally, the "new", Db2 12 TRANSFER OWNERSHIP command is implemented as a simple, immediate change.

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[1] Not all types of tables can be renamed. Consult the IBM Db2 SQL Reference manual, page 2163, for types of tables and options that forbid renaming a table.

[2] Adding a column at the end of a table requires that the column be nullable or have a default assigned, otherwise it is a complex change

[3] Can change data type within data type families (text to text, number to number, etc.)

[4] Can change length as an immediate change as long as it is larger, otherwise it is a complex change.

[5] With the caveat that the CHECK utility will have to be run to enforce a check constraint if the CURRENT RULES ‘DB2’ option is in effect

[6] When a materialized query table is dropped, all packages dependent on it are invalidated

Db2 for z/OS and Managing Database Changes - Part 1

Today we begin a multi-part series of blog posts taking a look at what is involved in making database changes in a Db2 for z/OS environment. The first thing that DBAs will need is the ability to change all the database objects supported by Db2 for z/OS. There are numerous different types of  database objects and structures that can be created and modified by DDL, and at one point or another, DBAs are called upon to create, alter, and drop every one of them.

But let’s dig a little deeper into what is required. Assume that you are a Db2 DBA who has been given a request to make several changes to database structures. The first thing you must do, of course, is to review the requested changes to make sure they are appropriate. Assuming they are, what is the next step?

You must determine how to go about making each change. At a high level, there are three different types of schema changes: 

• simple (or immediate), 
• medium (or pending), and 
• complex. 

Simple changes can be implemented immediately without requiring intervening actions. Medium changes require a bit more work to implement by running a REORG, and then we have complex changes that require an in-depth script for dropping and re-creating the database object. But not every type of database change request can use each type of schema change method. There are requirements and nuances in deciding which method can be used when.

In our next blog post, we will discuss simple Db2 changes.