Monday, November 23, 2009

Reading Things That Aren't There... and Missing Things That Are!

You can shoot yourself in the foot using DB2 if you are not careful. There are options that you can specify that may cause you to read data that is not really in the database. And, alternately, you can set things up so that you miss reading data that is actually in the database.

How, you might be asking? Well, dirty reads will take care of the first one. Specifying ISOLATION(UR) implements read-through locks, which is sometimes referred to as a dirty read. It applies to read operations only. With this isolation level data may be read that never actually exists in the database, because the transaction can read data that has been changed by another process but is not yet committed.

Read uncommitted isolation provides the highest level availability and concurrency of the isolation levels, but the worst degree of data integrity. It should be used only when data integrity problems can be tolerated. Certain types of applications, such as those using analytical queries, estimates, and averages are likely candidates for read uncommitted locking. A dirty read can cause duplicate rows to be returned where none exist or no rows may be returned when one (or more) actually exists. When choosing read uncommitted isolation the programmer and DBA must ensure that these types of problems are acceptable for the application.

OK, so what about not reading data that is in the database? DB2 V9 provides us an option to do just that. In DB2 9 it is possible for a transaction to skip over rows that are locked. This can be accomplished by means of the SKIP LOCKED DATA option within your SQL statement(s). SKIP LOCKED DATA can be specified in SELECT, SELECT INTO, and PREPARE, as well as searched UPDATE and DELETE statements. You can also use the SKIP LOCKED DATA option with the UNLOAD utility.

When you tell DB2 to skip locked data then that data is not accessed and your program will not have it available. DB2 just skip over any locked data instead of waiting for it to be unlocked. The benefit, of course, is improved performance because you will not incur any lock wait time. But it comes at the cost of not accessing the locked data at all. This means that you should only utilize this clause when your program can tolerate skipping over some data.

The SKIP LOCKED DATA option is compatible with cursor stability (CS) isolation and read stability (RS) isolation. But it cannot be used with uncommitted read (UR) or repeatable read (RR) isolation levels. DB2 will simply ignore the SKIP LOCKED DATA clause under UR and RR isolation levels.

Additionally, SKIP LOCKED DATA works only with row locks and page locks. That means that SKIP LOCKED DATA does not apply to table, partition, LOB, XML, or table space locks. And the bigger the lock size, the more data that will be skipped when a lock is encountered. With row locking you will be skipping over locked rows, but with page locking you will be skipping over all the rows on the locked page.

Use both of these features with extreme care and make sure that you know exactly what you are telling DB2 to do. Otherwise, you might be reading more... or less than you want!

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Tuesday, November 17, 2009

Replacing UNION with CASE

When a UNION is required to put together data from multiple queries, you might be able to use a CASE statement instead. This is very useful, particularly when the data for each of the queries in the UNION come from the same table. The CASE statement can potentially enhance performance by minimizing the number of times the data is read.

Let’s look at an example to clarify why:

SELECT CREATOR, NAME, 'TABLE'
FROM SYSIBM.SYSTABLES
WHERE TYPE = 'T'
UNION
SELECT CREATOR, NAME, 'VIEW '
FROM SYSIBM.SYSTABLES
WHERE TYPE = 'V'
UNION
SELECT CREATOR, NAME, 'ALIAS'
FROM SYSIBM.SYSTABLES
WHERE TYPE = 'A'
ORDER BY NAME;

This simple SQL statement uses UNION to put together the results of three queries against the SYSTABLES table. The report shows all of the DB2 table-like objects that exist in the DB2 subsystem: tables, views, and synonyms.

To do this, DB2 must scan through the table three times – once for each query (as there is no index on the TYPE column). But, you can use CASE and code an equivalent, but more efficient query, as follows:

SELECT CREATOR, NAME,
CASE TYPE
WHEN 'T' THEN 'TABLE'
WHEN 'V' THEN 'VIEW '
WHEN 'A' THEN 'ALIAS'
END
FROM SYSIBM.SYSTABLES
ORDER BY NAME;

This new query will need to scan SYSTABLES only once. The CASE statement will translate the code in the TYPE column into the text that we desire.

CASE statements are very powerful and you should use them when you can to create elegant and optimal SQL in your DB2 applications.

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Tuesday, November 03, 2009

Deprecated Features Planned for DB2 X for z/OS

Everyone is always interested in the latest and greatest features of their favorite DBMS, in this case DB2. But sometimes features get removed from the DBMS when a new version is released. According to the IBM teleconference on DB2 X for z/OS today, there are several features planned to be deprecated (i.e. removed). Let's briefly take a look at them.

The first feature that will be removed is private protocol DRDA. This should come as no surprise to anybody since IBM has been indicating that private protocal distribution was on its way out for a number of releases now. And it really is not that difficult to convert to DRDA (and there will be some new help in DSNTP2DP).

Perhaps more troublesome to some shops will be the removal of support for plans containing DBRMs. You will have to convert the DBRMs to packages when you get to DB2 X. You would think that since packages have been available as far back as V2.3 that most shops would have converted to them already. But I do know that there are some shops out there" who still have DBRMs bound directly into their plans. So start planning to convert ASAP! IBM will offer some support in DB2 X to track down affected plans.

While on the topic of plans and packages, you will have to REBIND any that have not been rebound since V5 or before. But it is a good rule to REBIND everything when you go to a new version of DB2 anyway, and most (but not all) shops do that.

From a documentation perspective, BookManager will no longer be supported. Instead we get the PDF versions and the Info Center online.

And the DB2 Management Clients (DB2 Administration Server, Control Center, and Development Center) are deprecated. IBM's new management client direction for DB2 is Data Studio.

Other deprecated items include:
  • ACQUIRE(ALLOCATE) [change to ACQUIRE(USE)]
  • Workload capture through profile monitor
  • XML Extender [change to use pureXML data type]
  • DB2 MQ XML user-defined functions and stored procedures [change to XML functions]
  • msys for Setup DB2 Customization Center [change to install panels]
At this point, DB2 X is on track to become generally available at the end of next year, 2010. At this point IBM has announced that migration will be supported from DB2 9 NFM (and has hinted that they are considering supporting migration to DB2 X directly from DB2 V8 NFM).

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Monday, November 02, 2009

New DB2 Twitter List

Just a very quick post this morning to let all you DB2 Twitter folks out there know that I created a list of the DB2 tweeters I know about at http://twitter.com/craigmullins/db2-folks.

If you are a DB2 professional and I left you off the list please leave a comment here or drop me an e-mail and I'll be happy to add you.

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