Monday, April 30, 2007

Database Definition on Demand [DB2 9 for z/OS]

As you probably know, online schema evolution (sometimes referred to as “online schema change”) was one of the key new features of DB2 V8. But, as its name implies, its capabilities continue to evolve. With V9, online schema evolution expands to simplify more types of database definition changes. The new term IBM is using for this in V9 is Database Definition On Demand (DDOD).

One of the nice new components provided by DDOD in V9 is that online table space reorganization is significantly improved. Today, when reorganizing just a couple of partitions in a partitioned table space the BUILD2 phase takes a long time to complete. V8 removed the outage for DPSIs, and now V9 removes the BUILD2 phase for all types of secondary indexes.

Another new DDOD capability supports replacing one table quickly with another. This is accomplished via cloning and the technique can even avoid the need to REBIND packages. Cloning allows you to generate, in the same table space, a new table with the same structure as the original table. After creating the clone you can do with it what you want – LOAD, DELETE, INSERT, UPDATE, etc. data – and then exchange the clone table name with the current table name. In this way you can keep the existing table operational while you work on the next “generation” of that table in its clone. When the clone is ready, you EXCHANGE it with the existing table. Nice, huh?

How about being able to rename a column within your table or rename an index? V9 provides the ability to do both! No longer do you have to DROP and re-CREATE in order to rename columns and indexes.

DB2 V9 also introduces a new type of table space that combines the attributes of segmented and partitioned. It is called a universal table space. Universal table spaces offer improved space management for variable length rows. This is so because it uses the space map page with more free space information like segmented table spaces. Also, like segmented table spaces, universal table spaces deliver improved mass delete performance and you can immediately reuse the table segments after the mass delete.

There are two types of universal table spaces:

  • Partition-by-growth: this type of universal table space will partition the data as it grows without the need to specify key ranges. This type of universal table space is beneficial for tables that will grow over time and/or need the additional limits afforded by partitioning, but can benefit from the performance of segmented. You can define more than one table in this type of universal table space if you wish.
  • Range-partitioned: this type of universal table space requires a key range for partitioning – and it can contain only a single table. This is basically adding segmentation to the existing partitioned table space.

Additionally, you can define SMS constructs (MGMTCLAS, DATACLASS, and STORCLAS) on a STOGROUP and you can ALTER those constructs as well. And table space and index logging parameters can be altered.

DB2 V9 even adds a new capability to change the DB2 early code without requiring an IPL.

So, with DB2 9 for z/OS we get more flexibility in modifying our database schemas. And that is a good thing, right?

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