Monday, February 13, 2017

The DB2 12 for z/OS Blog Series - Part 4: Real Time Statistics History

For those uninitiated to what RTS are, they are a series of statistics that are automatically maintained by DB2 and can be used by DBAs (and tools) to schedule maintenance tasks like REORG and COPY.

Prior to the introduction of RTS, the only way to gather statistics about DB2 database structures was by running the RUNSTATS utility. RUNSTATS collects statistical information about DB2 database objects and stores this data in the DB2 Catalog. There are two types of statistics collected by RUNSTATS: data used by the optimizer to formulate efficient access paths and data used by DBAs to determine when maintenance tasks should be run. The optimization statistics are still required, and therefore, so is RUNSTATS. The maintenance statistics, on the other hand, can be ignored and replaced by RTS.

RTS runs in the background and automatically updates statistics in two system catalog tables (one for table space data and one for index data) as your DB2 data is modified. This differs from RUNSTATS, which is a hands-on administrative process. RTS is hands-off.
So what is new with RTS in DB2 12 for z/OS?

The most significant enhancement is the ability to maintain RTS history using DB2’s system-time temporal capabilities. When history is enabled, both SYSIBM.SYSINDEXSPACESTATS and SYSIBM.SYSTABLESPACESTATS will have a table with "_H" on the end that have the same columns, with the same data types. But the temporal relationship is not automatically enabled, you have to choose to do so.

To enable the temporal relationship between a history table and its associated catalog table you must issue an ALTER statement to “hook up” the relationship. Here are the DDL statement for each of the RTS tables:



Before turning on RTS history you should take care to build a process for maintaining, consolidating and purging historical information. Remember, that each time the STATSINT interval is reached, new RTS data will be written, causing the old information to be written to the system time temporal table. That history table will grow over time. You will want to have a plan for how to manage that growth. Perhaps you will want to aggregate the data daily, or weekly, or monthly. Or maybe you will just want to purge the data after a period of time. At any rate, your strategy for managing this data should be worked out before you decide to start collecting RTS history.

Why would you want to record historical RTS information? Well, such details can be useful for analyzing activity and predicting future requirements. With historical RTS you can see trends and might be able to improve performance, maintenance tasks, and service to your end users. And, of course, this means that you can write SQL time travel queries against the RTS system catalog tables.

You can always turn off historical RTS collection by severing the temporal relationship. This is accomplished by issuing ALTER TABLE specifying the DROP VERSIONING clause on the appropriate table. For example:


At any rate, the ability to take advantage of DB2’s temporal capabilities to capture RTS history is a powerful new feature of DB2 12 for z/OS that DBAs and performance analysts will want to take advantage of.

One other DB2 12 change to RTS worth noting, is the addition of the GETPAGES column to both table space and index space stats. This column is used to record the number of GETPAGE requests for the table space since the last time REORG was run (or since the object was created).

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