Thursday, April 23, 2020

Db2 for z/OS and Managing Database Changes - Part 3

Welcome to the third installment of our series examining the types of database changes that can be performed using Db2 for z/OS. In part 1 we introduced the three types of changes and in part 2 we looked at simple changes. Today we will talk about the next type of change to consider, the medium or pending change.

A pending change requires a little more work than does a simple change, but is much easier to implement than a complex change. The pending change was introduced in DB2 10 and significantly simplifies some types of database change.

Pending changes are supported only for database objects in Universal table spaces. If a change must be made to a structure in a segmented or classic partitioned table space, you cannot use the pending change capability. Pending changes are made in a non-disruptive way using the ALTER statement to make the desired change, but requiring a REORG to drive the actual, underlying change to the database structures. Because a reorganization can be run online, pending changes can be implemented with little, to no downtime on the system. And changes are easier to back off; simply issue the DROP PENDING CHANGES command (as long as no REORG has been run).

With pending changes, Db2 semantically validates the request and checks authorization at execution time as usual, but the change is not actually implemented. It is simply registered in the Db2 Catalog in a table named SYSIBM.SYSPENDINGDDL. When the change is requested, the object goes into an advisory state, AREOR, and the ALTER statement returns an SQLCODE of +610 indicating that the object has been placed into a pending state, but it remains completely available to your applications.

So, as you make deferred ALTER changes Db2 will begin to populate the changes into the SYSIBM.SYSPENDINGDDL table. Each pending change will have a row in the table. Depending upon what you have changed, a single ALTER can produce multiple rows in the SYSPENDINGDDL table.

Your changes are recorded in SYSPENDINGDDL rows as they are made and then applied in that order. For example, you can convert a segmented table space to Universal PBG. And then modify the DSSIZE. These changes are recorded by DB2 in that order and allowed.

It is possible, too, to make multiple changes to the same parameter and have them build up in the pending table. Say that you change the buffer pool for a table space from BP0 to BP32K. And then later change the same table space to BP8K2 before you run a REORG. In this case, you will end up with the TS in the BP8K2 buffer pool and 8K page sizes. DB2 knows and maintains the order of your changes and will get it right when you implement the deferred changes using REORG.

The actual, underlying changes are only made by Db2 when you run the REORG utility using SHRLEVEL CHANGE or REFERENCE. Another way of thinking about this is that only when a Shadow object is being used will Db2 implement pending changes.  Of course, you can still run a REORG using SHRLEVEL NONE but none of your pending changes will be implemented (that is, the changes will still be pending and the Pending Status will not be reset). The REORG can be executed at either the table space or index level… keeping in mind that dependent index changes will be implemented by reorganizing the table space containing the table that the index is built on.

Db2 does not permit combining deferred and immediate ALTERs in a single SQL statement, so be careful about what you are trying to request. Additionally, most immediate ALTERs are not possible while changes are pending.

It is a good idea, though not a requirement, to avoid confusion by materializing pending changes as soon as possible. When you have an Advisory Reorg Pending (AREO*) status clean it up with a REORG as quickly as makes sense. And make sure that you do so before making new changes whenever possible. With multiple changes out there pending to be made, it can be confusing and you may have forgotten all that was requested before. Additionally, there can be performance degradation if you do not clean up that Advisory Reorg Pending (AREO*) status.

Examples of medium changes that can be implemented as pending include converting a segmented table space to a Universal partition-by-growth table space, converting a classic partitioned table space to a Universal partition-by-range table space[7], converting a Universal partition-by-growth table space to RPN[8], changing the DSSIZE[9] of a table space[10], SEGSIZE[11], increasing MAXPARTITIONS, changing MEMBER CLUSTER, dropping a column from a table[12], renaming a column[13], modifying partitioning and rotating partitions, and regenerating an index.

Additionally, as of Db2 12, there is a new capability to set a system parameter that will treat all ALTER COLUMN changes as pending, even though you can change the data type, length, precision, and scale as immediate changes.

Remember that all changes implemented as pending using deferred ALTER require Universal table spaces. For any other type of table space, they are treated as complex changes.

[7] The classic partitioned table space must be table-controlled, not index-controlled
[8] Using the PAGENUM RELATIVE parameter
[9] Although the change can be simple/immediate if the data sets have not yet been created and no pending changes have been requested.
[10] Although the change can be simple/immediate if the data sets have not yet been created, no pending changes have been requested or the specified buffer pool is the same size as the current buffer pool.
[11] There are conditions where this can be an immediate, simple change
[12] Some columns drops are not allowed without other changes or require a complex script to implement
[13] Renaming a column becomes a complex change if the column is referenced in a view, index, row permission, column mask, UDG, check constraint or FIELDPROC. The change is also complex if the table containing the column is or is referenced by an MQT, has a trigger, has a VALIDPROC or and EDITPROC with row attributes.

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