Dynamic SQL - Let's Do The Math

We've come a long way in the world of DB2 in the past decade or so. I remember way back when it was common for DBAs to say "If performance is an issue, dynamic SQL should not be used... always use static SQL!"  But today, in 2014, it is usually the case that dynamic SQL is the predominant form of new development.

Now a lot of things have changed to make this the case. Particularly that most new applications are being developed for distributed and web applications, instead of traditional mainframe, COBOL applications. And dynamic SQL is the default way to access DB2 from these type of apps.

But you know what? Even if you are developing traditional mainframe COBOL programs, dynamic SQL can be a better solution for you.

The Basics

Before we go on, let's tackle a few of the basics. What makes dynamic SQL different than static SQL?  Well, static SQL is optimized prior to program execution.  Each and every static SQL statement in a program is analyzed and optimized during the DB2 Bind process.  During this process the best access path is determined and coded into your DB2 package.  When the program is executed, the pre-formulated access path is executed.

Dynamic SQL, on the other hand, is optimized at run time.  Prior to the dynamic SQL statement being executed, it must be processed by the DB2 Optimizer so that an optimal access path can be determined.  This is called the Prepare process.  Prepare can be thought of as a dynamic Bind. 

We will not go into the details of dynamic statement caching and its ability to improve dynamic SQL performance here. Suffice it to say, dynamic SQL these days can be tuned using caching. For additional details on dynamic statement caching (and REOPT parms) check out my article, Dynamic SQL Performance, on TOAD World.

Now let's turn our attention to traditional dynamic SQL development. There are four types of dynamic SQL:

• EXECUTE IMMEDIATE
• Non-SELECT
• Fixed-List SELECT
• Varying-List SELECT

EXECUTE IMMEDIATE dynamic SQL ­will (implicitly) prepare and execute complete SQL statements embedded in host-variables.  Its drawbacks are that it can not be used to retrieve data using the SELECT statement and the PREPARE is implicit within the EXECUTE IMMEDIATE; so optimization must occur every time the statement is executed.

Non-SELECT Dynamic SQL can be used to explicitly prepare and execute SQL statements in an ­application program.  The PREPARE and EXECUTE are separated so that once a statement is prepared, it can be executed multiple time without re-optimization.  However, as its name implies, Non-SELECT dynamic SQL can not ­issue the SELECT statement. 

Fixed-List SELECT can be used to explicitly prepare and execute SQL SELECT statements ­where the exact columns to be retrieved are always known in advance.  The columns to be retrieved must be known at the time the program is being coded and they can not change during execution.  This is necessary ­in order to create the proper working-storage declaration for ­host-variables in your program. 

If you do not know in advance ­the exact columns that are to be accessed, you can use Varying-List SELECT dynamic SQL.  In this case, pointer variables are used to maintain the list of selected columns.  Although Varying-List SELECT is the most complicated type of dynamic SQL, it also provides the most flexibility for dynamic SELECT statements.  Changes can be made "on the fly" to tables,­ and columns, and predicates.  Because everything about the query­ can change during one invocation of the program, the number and­ type of host-variables needed to store the retrieved rows cannot­ be known beforehand.  This will add considerable sophistication and complexity to­ your application programs.

A Mathematical Reason to Reconsider Dynamic SQL

Even if the decreasing cost of dynamic SQL and the newer performance improvements like dynamic statement caching do not compel you to use dynamic SQL, there is at least one situation where dynamic SQL should almost always be chosen over static SQL:  when numerous combinations of predicates can be chosen by a user at run-time.

Consider the following:  What if, for a certain query, there are twenty possible predicates.  The user of the program is permitted to choose up to six of these predicates for any given request.  How many different static SQL statements need to be coded to satisfy these specifications?

First, let's determine the number of different ways that you can choose six predicates out of twenty.  To do so we need to use combinatorial coefficients.  So, if n is the number of different ways then:

            n = (20 x 19 x 18 x 17 x 16 x 15) / (6 x 5 x 4 x 3 x 2 x 1)

            n = (27,907,200) / (720)

            n = 38,760

38,760 separate static SELECTs is quite a large number, but this is still not enough!  This number shows the total number of different ways we can choose six predicates out of twenty if the ordering of the predicates does not matter (which for all intents and purposes it does not)[1].  However, since the specifications clearly state that the user can choose up to six, we have to modify our number.  This means that we have to add in:

• the number of different ways of choosing five predicates out of twenty
• the number of different ways of choosing four predicates out of twenty
• the number of different ways of choosing three predicates out of twenty
• the number of different ways of choosing two predicates out of twenty
• the number of different ways of choosing one predicate out of twenty

Figure 1.  Combinatorial Coefficients Calculations

Ways to Choose Six Predicates Out of Twenty

            (20 x 19 x 18 x 17 x 16 x 15) / (6 x 5 x 4 x 3 x 2 x 1) = 38,760

Ways to Choose Five Predicates Out of Twenty

            (20 x 19 x 18 x 17 x 16) / (5 x 4 x 3 x 2 x 1) = 15,504

Ways to Choose Four Predicates Out of Twenty

            (20 x 19 x 18 x 17) / (4 x 3 x 2 x 1) = 4,845

Ways to Choose Three Predicates Out of Twenty

            (20 x 19 x 18) / (3 x 2 x 1) = 1,140

Ways to Choose Two Predicates Out of Twenty

            (20 x 19) / (2 x 1) = 190

Ways to Choose One Predicate Out of Twenty

            20 / 1 = 20

Total Ways to Choose Up To Six Predicates Out of Twenty

            38,760 + 15,504 + 4,845 + 1,140 + 190 + 20 = 60,459

This brings the grand total number of static SQL statements that must be coded to 60,459.  The calculations are shown in Figure 1.  In a situation like this, if static SQL is being forced upon us, we have one of two options:

1.   code for forty days and forty nights hoping to successfully write 60,459 SQL statements

2.   compromise on the design and limit the users flexibility

I can guarantee that 99.99% of the time the second option will be chosen.  My solution would be to abandon static SQL and use dynamic SQL in this situation.  How would this ease the development situation?  Consider the following:

• With dynamic SQL, the twenty predicates need be coded only once (in working storage)
• As the program runs, the application logic can build the complete SQL statement based upon user input
• The size of the DBRM will decrease dramatically.  The DBRM for the static SQL program will be huge if it contains all of the 60,459 SQL statements.  Even if a compromise number is reached, chances are that the DBRM will be large.  And guaranteed it will be larger than the DBRM for the dynamic SQL program.
• Although there will be additional run-time overhead to build the SQL and perform the dynamic Prepare, performance may not suffer. Queries issued against non-uniform data, may actually experience improved access paths and perform better.

So When Should You Seriously Consider Dynamic SQL?

• When the nature of the program is truly changeable, such as the example given in the text above.
• When the columns to be retrieved can vary from execution to execution.  This is similar to the example given where multiple combinations of predicates can be chosen, but in this case, multiple combinations of columns can be chosen.
• When benefit can be accrued from interacting with other dynamic SQL applications.  
• When the SQL must access non-uniform data.

You can find some additional guidance for helping you to evaluate when to use dynamic versus static SQL in my Data Perspectives column Dynamic vs. Static SQL.

Synopsis

Dynamic SQL is not always bad... and it is already pervasive in distributed and web applications.  In this day and age, dynamic SQL should be considered as a viable option even for traditional mainframe applications that are not distributed or web-based.

---

[1] It is true that for performance reasons you may want to put the predicate with the highest cardinality within each type of operator first, but we will not concern ourselves with this in this blog post.

Rebinding for Optimal DB2 Access Paths

The access paths formulated by the DB2 optimizer during the BIND and REBIND processes are critical to your application performance. It is these access paths that determine not only how DB2 data is accessed by your program, but how efficiently it is accessed. Whether you are preparing a new program, implementing changes into your existing DB2 applications, upgrading to a new version of DB2, or simply trying to achieve optimum performance for existing applications, an exhaustive and thorough REBIND management policy should be of paramount importance.

However, many organizations are not doing everything possible to keep access paths up-to-date with the current state of their data. So what is the best practice approach for rebinding your DB2 programs? The answer is “The Five R’s.” This methodology of regular rebinding followed by a review of your access paths required the following steps:

1.       Start with an inspection of the RTS (Real Time Statistics) to determine which objects need to be reorganized.
2.       Follow that up by running a REORG on table spaces and indexes as appropriate based on the statistics.
3.       After reorganizing, run RUNSTATS (to ensure the DB2 Catalog is up-to-date).
4.       Follow that with REBINDs of your programs.
5.       Then we need that fifth R – which is to review the access paths generated by the REBIND.

For shops that have avoided rebinding for years this approach represents a significant change. So what new DB2 features are available to help? Well, several releases ago, back in DB2 9 for z/OS, plan stability was added. This feature enables you to save a backup version of your access paths as a precautionary measure. If any of the new access paths after rebinding are less efficient, the DBA can switch back to the backed up access paths. In order to implement this level of stability you can use the PLANMGMT parameter of the REBIND command. There are three options: NONE, BASIC, and EXTENDED. BASIC saves the previous access paths, and EXTENDED saves the previous and an original. You can use REBIND and the SWITCH parameter to revert back to the saved access paths when the new access paths cause degraded performance. 

As of DB2 10 for z/OS you can tell DB2 to try to reused previous access paths for SQL statements whenever possible. This is called access path reuse and is implemented using the APREUSE bind option. When invoked, DB2 uses information about the previous access paths to create a hint.

When BIND PACKAGE or REBIND PACKAGE specifies APREUSE(ERROR), DB2 tries to locate the access path information from a package that has a matching identity. If no such package exists, DB2 tries to locate another recent version of the package that has the matching location, collection ID, and name. The APREUSE option only applies to statements that have identical statement text in both packages. Newly added statements and statements with text changes never reuse previous access paths.

Reusing access paths can help to minimize administrative and performance issues involved in rebinding.

Of course, there are products on the market which can be used to implement a proactive approach to rebinding. These products preview the new access paths and then run them through a rules system to determine if the new access will be improved, unchanged, or degraded. With this information we can rebind everything that would improve and avoid rebinding anything else until we can analyze the cause of the degradation. Using such an approach you should not have degraded access paths sneaking into your production environment.

Summary

At any rate, a systematic approach to DB2 binding and rebinding is necessary to assure optimal performance within your DB2 applications. This short blog entry covers some of the basics and recent changes to DB2 in this area. 

Be sure to take your time and to plan your statistics-gathering and bind/rebind approach... or be ready to be in constant firefighter mode as you address poorly-performing SQL in your applications!

Those Good Old IBM Mainframe Utility Programs

Most mainframe programmers are aware that IBM supplies many utility programs that are useful for system maintenance chores such as copying and deleting files, manipulating partitioned data sets, and the like.  

These utilities typically begin with an IEB, IEF, or IEH prefix.  One of the most common of these is IEFBR14, which is used to catalog a data set.  But few people are aware, however, that IBM also supplies many other utilities that can make the life of a programmer much easier.  Below is a select list of these:

IEBMAGIC        fixes any problem without having to use your brain; perfect for consultants and contract programmers

IEBIQUIT         automatically updates your resume, writes a letter of resignation, forwards it to your boss, and prints reams of paper to make it look like you actually did something when you worked here

IEBIBALL         compares any source file to the last one that actually worked displaying all changes and who made them; perfect tool for technical support personnel overwhelmed by programmers chanting the phrase "I didn't change anything"

IEBPANIC         if all else fails, run IEBPANIC; sometimes it fixes your problem and sometimes it doesn't, but it never tells you how it did it; companion program to IEBMAGIC;

IEBNOTME      alters all trace of your userid from system change logs, SMF, etc.; useful to eliminate finger-pointing; this should always be run before running IEBIQUIT

IEFINGER        when designing on-line systems sometimes intermediate screens are sent that serve no purpose other than to tie together a series of related transactions; these intermediate screens generally direct the user to "Press ENTER to Continue"; IEFINGER simulates the end user pressing the ENTER key thereby eliminating unnecessary screens

IEHAMMER      forces a square peg into a round hole; for example, if you try to compile a COBOL program using the FORTRAN compiler, attaching IEHAMMER to the compile will make it work

IEBPIG               finds all unused resources of any type and assigns them to any specified job

IEBHAHA         randomly changes source code in test libraries; it is often speculated that IEBHAHA is the cause of most program problems that IEBIBALL is used to correct

IEBEIEIO          run this utility when you have too many problems to handle for one person;  it corrects the old "with an abend here, and a meeting there, e-i-e-i-o" syndrome by causing a system problem so large (in someone else's application) that all direction is diverted away from you to them

So did I forget your favorite?  Drop a comment below to share it!

The Twelve DBA Rules of Thumb... a summary

Over the past couple of months this blog has offered up some rules of thumb for DBAs to follow that can help you to build a successful and satisfying career as a DBA. These twelve rules of thumb worked well for me as I worked my way through my career and I have shared them with you, my faithful readers, so that you can benefit from my experiences. I hope you find them useful... and if I have missed anything, please post a comment with your thoughts and experiences on being a good DBA.

As a reminder of what we have discussed, I am posting a short synopsis of the Twelves DBA Rules of Thumb here, along with links to each blog post.

 1. Write Down Everything
 2. Automate Routine Tasks
 3. Share Your Knowledge
 4. Analyze, Simplify and Focus
 5. Don't Panic!
 6. Be Prepared
 7. Don't Be a Hermit
 8. Understand the Business, Not Just the Technology
 9. Ask for Help When You Need It
10. Keep Up-to-Date
11. Invest in Yourself
12. Be a Packrat


Good luck with your career as a DBA...

DBA Rules of Thumb - Part 12 (Be a Packrat)

Today's post in the DBA Rules of Thumb series is short and sweet. It can be simply stated as "Keep Everything!"

Database administration is the perfect job for you if you are a pack rat.

It is a good practice to keep everything you come across during the course of performing your job. When you slip up and throw something away, it always seems like you come across a task the very next day where that stuff would have come in handy... but you you threw it out!

I still own some printed manuals for DB2 Version 2. They are packed up in a plastic tub in my garage, but I have them in case I need them.

DBA Rules of Thumb - Part 11 (Invest in Yourself)

Most IT professionals continually look for their company to invest money in their ongoing education. Who among us does not want to learn something new — on company time and with the company’s money? Unless you are self-employed, that is!

Yes, your company should invest some funds to train you on new technology and new capabilities, especially if it is asking you to do new things. And since technology changes so fast, most everyone has to learn something new at some point every year. But the entire burden of learning should not be placed on your company.

Budget some of your own money to invest in your career. After all, you probably won’t be working for the same company your entire career. Why should your company be forced to bankroll your entire ongoing education? Now, I know, a lot depends on your particular circumstances. Sometimes we accept a lower salary than we think we are worth because of the “perks” that are offered. And one of those perks can be training. But perks have a way of disappearing once you are "on the job."

Some folks simply abhor spending any of their hard-earned dollars to help advance their careers. This is not a reasonable approach to your career! Shelling out a couple of bucks to buy some new books, subscribe to a publication, or join a professional organization should not be out of the reach of most DBAs.

A willingness to spend some money to stay abreast of technology is a trait that DBAs need to embrace. 

Most DBAs are insatiably curious, and many are willing to invest some of their money to learn something new. Maybe they bought that book on NoSQL before anyone at their company started using it. Perhaps it is just that enviable bookshelf full of useful database books in their cubicle. Or maybe they paid that nominal fee to subscribe to the members-only content of that SQL Server portal. They could even have forked over the $25 fee to attend the local user group.

Don’t get me wrong. I’m not saying that companies should not reimburse for such expenses. They should, because it provides for better-rounded, more educated, and more useful employees. But if your employer won’t pay for something that you think will help your career, why not just buy it yourself?

And be sure to keep a record of such purchases because unreimbursed business expenses can be tax deductible. 

DBA Rules of Thumb - Part 10 (Keep Up-to-Date)

If you wish to be a successful DBA for a long period of time, you will have to keep up-to-date on all kinds of technology — both database-related and other.

Of course, as a DBA, your first course of action should be to be aware of all of the features and functions available in the DBMSs in use at your site — at least at a high level, but preferably in depth. Read the vendor literature on future releases as it becomes available to prepare for new functionality before you install and migrate to new DBMS releases. The sooner you know about new bells and whistles, the better equipped you will be to prepare new procedures and adopt new policies to support the new features.

Keep up-to-date on technology in general, too. For example, DBAs should understand new data-related technologies such as NoSQL, Hadoop, and predictive analytics, but also other newer technologies that interact with database systems. Don’t ignore industry and technology trends simply because you cannot immediately think of a database-related impact. Many non-database-related “things” (for example, XML) eventually find their way into DBMS software and database applications.

Keep up-to-date on industry standards — particularly those that impact database technology such as the SQL standard. Understanding these standards before the new features they engender have been incorporated into your DBMS will give you an edge in their management. DBMS vendors try to support industry standards, and many features find their way into the DBMS because of their adoption of an industry standard.

As we've already discussed in this series, one way of keeping up-to-date is by attending local and national user groups. The presentations delivered at these forums provide useful education. Even more important, though, is the chance to network with other DBAs to share experiences and learn from each other’s projects.

Through judicious use of the Internet and the Web, it is easier than ever before for DBAs to keep up-to-date. Dozens of useful and informative Web sites provide discussion forums, script libraries, articles, manuals, and how-to documents. Consult my web site at http://www.craigsmullins.com/rellinks.html for a regularly-updated  list of DBMS, data, and database-related Web resources.

Remember, though, this is just a starting point. There are countless ways that you can keep-up-to-date on technology. Use every avenue at your disposal to do so, or risk becoming obsolete.

DBA Rules of Thumb - Part 9 (Call on Others for Help When Needed)

Use All of the Resources at Your Disposal

Remember that you do not have to do everything yourself. Use the resources at your disposal. We have talked about some of those resources, such as articles and books, Web sites and scripts, user groups and conferences. But there are others.

Do not continue to struggle with problems when you are completely stumped. Some DBAs harbor the notion that they have to resolve every issue themselves in order to be successful. Sometimes you just need to know where to go to get help to solve the problem. Use the DBMS vendor’s technical support, as well as the technical support line of your DBA tool vendors. Consult internal resources for areas where you have limited experience, such as network specialists for network and connectivity problems, system administrators for operating system and system software problems, and security administrators for authorization and protection issues.

As a DBA you are sometimes thought of as "knowing everything" (or worse a know-it-all), but it is far more important to know where to go to get help to solve problems than it is to try to know everything there is to know. Let's face it... it is just not possible to know everything about database systems and making them work with all types of applications and users these days.

When you go to user groups, build a network of DBA colleagues whom you can contact for assistance. Many times others have already encountered and solved the problem that vexes you. A network of DBAs to call on can be an invaluable resource (and no one at your company even needs to know that you called for outside help).

Finally, be sure to understand the resources available from your DBMS vendors. DBMS vendors offer their customers access to a tremendous amount of useful information. All of the DBMS vendors offer software support on their Web sites. Many of them provide a database that users can search to find answers to database problems. IBM customers can use IBMLink,^[1] and both Oracle and Microsoft offer a searchable database in the support section of their Web sites. Some DBAs claim to be able to solve 95 percent or more of their problems by researching online databases. These resources can shrink the amount of time required to fix problems, especially if your DBMS vendor has a reputation of “taking forever” to respond to issues.

Of course, every DBA should also be equipped with the DBMS vendor’s technical support phone number for those tough-to-solve problems. Some support is offered on a pay-per-call basis, whereas other times there is a prepaid support contract. Be sure you know how your company pays for support before calling the DBMS vendor. Failure to know this can result in your incurring significant support charges.

---

[1]. IBMLink is a proprietary network that IBM opens up only to its customers.

DBA Rules of Thumb - Part 8 (Being Business Savvy)

Understand the Business, Not Just the Technology

Remember that being technologically adept is just a part of being a good DBA. Although technology is important, understanding your business needs is more important. If you do not understand the impact on the business of the databases you manage, you will simply be throwing technology around with no clear purpose.

Business needs must dictate what technology is applied to what database—and to which applications. Using the latest and greatest (and most expensive) technology and software might be fun and technologically challenging, but it most likely will not be required for every database you implement. The DBA’s tools and utilities need to be tied to business strategies and initiatives. In this way, the DBA’s work becomes integrated with the goals and operations of the organization.

The first step in achieving this needed synergy is the integration of DBA services with the other core components of the IT infrastructure. Of course, DBAs should be able to monitor and control the databases under their purview, but they should also be able to monitor them within the context of the broader spectrum of the IT infrastructure—including systems, applications, storage, and networks. Only then can companies begin to tie service-level agreements to business needs, rather than technology metrics.

DBAs should be able to gain insight into the natural cycles of the business just by performing their job. Developers and administrators of other parts of the IT infrastructure will not have the vision into the busiest times of the day, week, quarter, or year because they are not privy to the actual flow of data from corporate business functions. But the DBA has access to that information as a component of performing the job. It is empowering to be able to understand business cycle information and apply it on the job.

DBAs need to expand further to take advantage of their special position in the infrastructure. Talk to the end users — not just the application developers. Get a sound understanding of how the databases will be used before implementing any database design. Gain an understanding of the database’s impact on the company’s bottom line, so that when the inevitable problems occur in production you will remember the actual business impact of not having that data available. This also allows you to create procedures that minimize the potential for such problems.

To fulfill the promise of business/IT integration, it will be necessary to link business services to the underlying technology. For example, a technician should be able to immediately comprehend that a service outage to transaction X7R2 in the PRD2 environment means that regional demand deposit customers cannot access their accounts. See the difference?

Focusing on transactions, TP monitors, and databases is the core of the DBA’s job. But servicing customers is the reason the DBA builds those databases and manages those transactions. Technicians with an understanding of the business impact of technology decisions will do a better job of servicing the business strategy. This is doubly true for the DBA’s manager. Technology managers who speak in business terms are more valuable to their company.

Of course, the devil is in the details. A key component of realizing effective business/IT integration for DBAs is the ability to link specific pieces of technology to specific business services. This requires a service impact management capability—that is, analyzing the technology required to power each critical business service and documenting the link. Technologies exist to automate some of this through event automation and service modeling. Such capabilities help to transform availability and performance data into detailed knowledge about the status of business services and service-level agreements.

Today’s modern corporations need technicians who are cognizant of the business impact of their management decisions. As such, DBAs need to get busy transforming themselves to become more business savvy — that is, to keep an eye on the business impact of the technology under their span of control. 

Happy New Year

Here we are at the end of another year and on the brink of a shiny New Year. Let's take this time to look back on the successes of 2013... and to examine our failures with an eye toward avoiding them in 2014.

Celebrate safely tonight... and let's all meet back here later this week to continue our series on DBA Rules of Thumb!

Happy New Year everybody!

Seasons Greetings

Just a short post today to wish all of my readers a very happy holiday season and to let you know that I will not be posting anything new between now and the end of the year...

But be sure to check back again in 2014 as I continue to write about DB2 and database issues that impact all of us!