Monday, April 22, 2019

Db2 Application Testing Considerations


Testing application programs is a discipline unto itself and there are many considerations and nuances to be mastered to be able to test software appropriately. This blog post will not go into a lot of depth regarding testing practices, other than to highlight the important things to keep in mind to assure optimal performance of your Db2 applications.

Statistics

The data that you use in your test environment will not be the same as your production data. Typically, you will have less test data than you do in production. So, if you run the RUNSTATS utility on your test data you will get different statistics than in production. 

Instead of running RUNSTATS, you can test your SQL access paths by updating the test catalog statistics to be the same as your production system. Do not attempt to modify test statistics on your own. You should work with your DBA group to set up a process for up­dating test statistics. This can be accomplished in various ways. Your organization may have a tool that makes it easy to copy statistics from production to test; or your DBA team may use a DDL script of queries and modification statements to populate test statistics from production.

If your test table definitions are different in test and produc­tion you will need to take this into account in the script. For example, things like creator, name, indexes, number of parti­tions, and even columns can differ between environments. Furthermore, you may have new tables and columns for which there are no current production statistics, meaning that you will need to create estimated statistics based on your knowledge of the business and application.

Some organizations make the mistake of copying production statistics to test once, and never (or rarely) populating test again. This is a mistake because most production databases change over time, sometimes dramatically. When you run Runstats for your production applications it is a good idea to also update your test statistics from the new production statis­tics.

Modeling a Production Environment in Test

Another tactic you can use to improve the accuracy of access path testing is to model the configuration and settings of your productionenvironment in your test system. Remember that the Db2 optimizer does not just use statistics, but also infor­mation about your computing environment.

Db2 test systems typically vary from the production system. Application testing is often accomplished on test systems that have different parameters and configurations than the produc­tion systems that run the applications. Test systems usually get set up on a less powerful processor (or LPAR), and use less memory for buffering, sorting, and other system processes. This can result in different access paths for test and produc­tion systems, which can cause performance problems that only show up after you move to production.

However, it is possible to model the configuration and param­eters of your production environment in your test system. You can specify configuration details for Db2 to use for access path selection in your test system using profile tables.

Work with your DBA group to configure proper profile settings for testing your applications.

Test Cases for Skewed Data

Db2 assumes that data values are mostly uniformly distributed throughout the data. However, not all data is uniformly distributed. Db2 RUNSTATS can capture information about non-uniformly distributed and skewed data.

When data is non-uniformly distributed a subset of the values occur much more frequently than others. A special case of non-uniformly distributed data is skewed data. When data is skewed, one value (or a very small number of values) occurs much more frequently than others.

Non-uniformly distributed and skewed data presents a performance testing challenge. The Db2 optimizer can formulate different access paths for non-uniformly distributed data based on the actual values supplied. This is particularly important for dynamic SQL applications, but you should be aware of non-uniformly distributed and skewed data even for static SQL applications.

For non-uniformly distributed data you can examine the Db2 catalog to obtain values for the most commonly occurring values. For Db2 for z/OS this information is in the SYSIBM.SYSCOLDIST table.

Be sure to test with several of the values that are stored in the Colvalue column of these tables, and some that are not. This will enable you to test the performance of the most common values and less common values. The access paths may differ for each and the performance also can vary.

An Example

Let’s discuss an example. Suppose you operate a bar with a cigar room and you want to keep track of customers with a table of cigar smokers. We gather information like name, address, phone number, sex, and favorite cigar in the table. Cigar smokers skew male, so there will likely be many more rows where Sex is M, than there are where Sex is F.  With that background, consider a query like this one:

  SELECT name, phoneno, fav_cigar
  FROM   cigar_smokers
  WHERE  sex = ?;

Because the data is skewed, it is possible that Db2 will choose a different access path for M than for F. If the vast majority of the rows are men, then a table scan might be chosen for Sex = ‘M’; whereas with only a few rows for women, an index might be chosen if one exists on the Sex column.

This is just a simple example. You need to understand your data and how it skews to make sure that you create and test sample test cases for all of the pertinent values.

SQL Variations

A final performance testing consideration is to consider multiple SQL variations, especially for queries that access a lot of data or have complex access paths. Do not just find one SQL formulation that works and stick with it. Remember from earlier chapters that you can code multiple variations of SQL statements that return the same data, but that perform quite differently.


Note
This blog post was adapted and excerpted from my latest book, A Guide to Db2 Performance for Application Developers. Click the link for more information or to buy a copy (both print and ebook available).



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