Temporal tables and time-travel query support was added in DB2 10 for z/OS. With temporal tables a time period is attached to the data to indicate when it was valid or changed in the database. A traditional database stores data implied to be valid at the current point-in-time; it does not track the past or future states of the data. Temporal support makes it possible to store different database states and to query the data as of those different states. That means different DDL to support temporal data, as well as different SQL syntax to query it.
Using built-in DB2 temporal capabilities is much easier than coding for it yourself, but as with most new features there were some additional tweaks that customers asked for. DB2 12 for z/OS delivers on making it easier for organizations to adopt and use temporal DB2 data and queries by supporting:
- temporal referential constraints,
- more flexible time periods, and
- logical transactions.
Let’s examine each of these areas in a bit more depth. First up, let’s take a look at temporal referential constraints. Prior to DB2 12, temporal referential integrity required triggers or stored procedures to implement. But with DB2 12, you can define a temporal referential constraint for a temporal table that contains a BUSINESS_TIME period by specifying the PERIOD BUSINESS_TIME clause in the definition of the constraint. This should improve data integrity for temporal tables and increase temporal uptake for DB2 customers.
The second new temporal feature is improved flexibility for defining the application period in temporal tables. The application period is defined with two date/time columns, one specifying the beginning of the period and the other specifying the end of the period. Prior to DB2 12, the beginning value of a period had to be inclusive, and the end value had to be exclusive. So for a period beginning at 2017-01-01 and ending 2019-10-01, 2017-01-01 is part of the period but 2019-10-01 is not part of the period. This type of period is called an inclusive-exclusive period.
In DB2 12, you can create an application-period temporal table with a BUSINESS_TIME period that is inclusive-inclusive. That means that the end value is considered to be part of the period, instead of outside the period definition.
Generally speaking, the inclusive-exclusive definition is preferred (and is the default) because it complies with the SQL standard. However, if your data already exists as inclusive-inclusive, or it makes more sense to your business users, then DB2 12 support for inclusive-inclusive will make it easier for your organization to use DB2 temporal tables and time travel queries.
Finally, DB2 12 offers temporal logical transactions on system temporal tables (that is, those using SYSTEM_TIME periods). With this new feature, DB2 supports logical units of temporal work that are not determined by COMMIT and ROLLBACK. That means that values for row-begin and row-end columns are determined by applications based on a built-in global variable that you can set.
So overall, DB2 12 for z/OS makes using temporal tables easier and more flexible with these three compelling new features.