Happy Thanksgiving 2017

Today, November 23rd, in the United States of America, we celebrate Thanksgiving by gathering together with our loved ones and giving thanks for what we have. 

Typically, this involves celebrations with food, traditionally a big turkey dinner with stuffing, mashed potatoes and gravy, as we watch a parade and football games. I plan to follow this tradition to the letter this year and I wish you the same!

And with the general notion of giving thanks on this fine day, I want to also pause and thank each and every one of you for reading my blog, whether just today, or all year long. Hopefully you'll keep coming back and we can keep discussing all things Db2 here.

But for today, wherever you may be, I am thankful for you and I wish you a very happy day!

Oh... and try not to get trampled on Friday if you're going out shopping for bargains!

The Db2 12 for z/OS Blog Series - Part 19: Profile Monitoring Improvements

The ability to monitor Db2 using profile tables is a newer, though by no means brand new capability for Db2 DBAs. You can use profile tables to monitor and control various aspects of Db2 performance such as remote connections and certain DSNZPARMs.

But this blog post is not intended to describe what profile monitoring is, but to discuss the new capabilities added in Db2 12 to enhance profile monitoring.

There are four new enhancements offered by Db2 12 for the use of system profiles.

The first enhancement is the ability to automatically start profiles when you start up a Db2 subsystem. This can be accomplished using a new subsystem parameter called PROFILE_AUTOSTART. Setting the parameter to YES causes Db2 to automatically execute START PROFILE command processing. The default is NO, which means that Db2 will not initiate START PROFILE when the subsystem starts up.

The second improvement is the addition of support for global variables. As of Db2 12 you can specify the following global variables as a KEYWORDS column value in the SYSIBM.DSN_PROFILE_ATTRIBUTES table:

• GET_ARCHIVE
• MOVE_TO_ARCHIVE
• TEMPORAL_LOGICAL_TRANSACTION_TIME
• TEMPORAL_LOGICAL_TRANSACTIONS

If a profile filter matches a connection, Db2 will automatically apply the built-in global variable value to the Db2 process of that connection when the connection is initially established, and when a connection is reused.

Wildcarding support is the third enhancement for profiles in Db2 12. One row for each profile is contained in the SYSIBM.DSN_PROFILE_TABLE. Each column in the table informs Db2 which connection to monitor. Without wildcarding, handling various connections required multiple rows to be defined in the table. But with Db2 12, you can have one row representing more than one connection. Wildcarding is available for AUTHID (authorization IDs), LOCATION (IP addresses of monitored connections), and PRDID (product specific identifier, for example DSN for Db2).

The fourth and final enhancement is for managing idle threads. The MONITOR IDLE THREADS column in the SYSIBM.DSN_PROFILE_ATTRIBUTES table directs DB2 to monitor (for an approximate amount of time) an active server thread’s idle time. The ATTRIBUTE1 column, which is used to specify the type of messages and level of detail of messages issued for monitored threads, has been enhanced to allow the following values: 

• EXCEPTION_ROLLBACK
• EXCEPTION_ROLLBACK_DIAGLEVEL1
• EXCEPTION_ROLLBACK_DIAGLEVEL2 

Note: This particular change to idle threads
for EXCEPTION_ROLLBACK was made available
in Db2 11 after general availability, and will be
available on a Db2 12 system after new function
is activated.

For more details on any of these capabilities, or indeed, on profile monitoring in general, refer to the IBM Db2 12 for z/OS Managing Performance manual, SC27-8857.

Db2 Social Advocate of the Year

A great big thank you to IBM and IDUG for naming me Db2 Social Advocate of the Year for 2017 at the Db2 Awards ceremony at IDUG in Lisbon this year.

I received a nice crystal award and recognition on stage at the event with Surekha, Daniel and Namik.

It is always great to be recognized for my contributions to the Db2 community, but it is something that I do because I love Db2... and have ever since I first used Version 1 back in the 1980's! 

So a big thank you to everyone involved and hopefully I can continue to warrant such accolades throughout the remainder of this year and for many years to come.

Long live Db2!

The Db2 12 for z/OS Blog Series - Part 18: Adaptive Indexes

Have you ever had one of those tough queries that was always a challenge to keep performing well? This type of query usually experiences fluctuating filtering. By that I mean that the filtering can change, sometimes dramatically, between executions of the query.

Some of the things that can cause fluctuating filtering are predicates with ranges that vary, sometimes returning a small subset of rows and sometimes returning everything. You know the type, perhaps there is a BETWEEN clause that can be set and sometimes it is set as BETWEEN 3 AND 5, whereas other times it is set as BETWEEN 0 and 999999. And maybe even sometimes it is set to BETWEEN 3 AND 3 to just search for equality... Or perhaps it is a LIKE clause that sometimes starts with a wildcard ('%').

Well, Db2 12 offers execution time adaptive indexes that allows list-prefetch plans to quickly determine filtering and adjust at execution time as needed. Db2 can do this for static SQL queries even if REOPT(ALWAYS) is not specified. 

Execution time adaptive indexes are not limited to search screening, as described in the previous paragraph. Indeed, any query with a high uncertainty in the optimizer’s estimate can benefit. This includes range predicates, JSON, Spatial, and index on expression queries.

A quick evaluation is performed by looking done at the literals used in the query. Further costlier evaluation of filtering is deferred until after one RID block is retrieved from all participating indexes. This offers a better optimization opportunity while at the same time minimizing overhead for short running queries.

How about some examples of how execution time adaptive indexes work? For an access path that uses list prefetching or a multi-index OR the query can fall back to a table space scan if a large percentage of the data is going to be read. For an access path that uses multi-index AND Db2 can reorder index legs from most to least filtering, as well as providing an early-out for non-filtering legs and fallback to a table space scan if there is no filtering.

If you are interested in tracking when adaptive index processing is utilized, IFCID 125 has been enhanced to track this feature.

The Db2 12 for z/OS Blog Series - Part 17: A New Privilege for UNLOAD

Db2 12 for z/OS introduces a new privilege that, when granted, enables a user to be able to unload data using the DB2 IBM UNLOAD utility. In past releases, the SELECT privilege (or other higher level admin privileges) was required to unload data using the UNLOAD utility. But this was less than desirable.

Why? Well, one reason is that it created a potential security gap. Consider the situation where a table has column masks or row permissions. In such as case, a user with SELECT privilege against the table still might not be able to access all of the rows and columns because of the masks/permissions that are defined. However, the same user with the same privilege set could execute the UNLOAD utility and be able to read all of the data in the table. Such as situation is not ideal and would not pass an audit.

To remove this gap IBM has introduced a new privilege, the UNLOAD privilege. After you move to Db2 12 for z/OS, SELECT authority is no longer enough to be able to unload data. In order to unload data the user must be granted the UNLOAD privilege on that table. The UNLOAD privilege can only be granted on a table; it cannot be granted on an auxiliary table or a view. The UNLOAD privilege is required after you have moved to function level V12R1M500 or higher.

Of course, there is a workaround if you still want to allow users with the SELECT privilege to be able to unload using the UNLOAD utility. This requires setting a DSNZPARM named AUTH_COMPATIBILITY to "SELECT_FOR_UNLOAD". The default for this DSNZPARM is NULL, which means that the UNLOAD privilege is required. 

Regardless of the privilege, keep in mind that tables with multilevel security impose restrictions on the output of your UNLOAD jobs. A row will be unloaded only if the security label of the user dominates the security label of the row. So it is possible that an unload may not actually unload every row in the table. If security label of the user does not dominate the security label of the row, the row is not unloaded and DB2 does not issue an error message.