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The last profiling technique included in the MySQL system is the ability to examine how the optimizer performs queries. While not strictly a performance-measuring device, it can be used to diagnose tricky queries that show up in the slow query log. As a simple example, let s see what the optimizer predicts about how the following query will be executed: select * from customer where phone like "%575%" This query is not very interesting and using the LIKE clause with %s surrounding the value is not efficient and almost sure to result in an index-less access method. If you run the command preceded by the EXPLAIN keyword, you see the results of the proposed query optimization. Listing 4-9 shows the results of using the EXPLAIN command. Listing 4-9. Output of EXPLAIN Command mysql> explain select * from customer where phone like "%575%" \G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: customer type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 599 Extra: Using where 1 row in set (0.00 sec)

The output shows that the command is a simple select on the customer table, there are no possible keys to use, there are 599 rows in the table, and the optimizer is using the WHERE clause. In this case, it is telling us that the execution will be a simple table scan without indexes perhaps one of the slowest possible select statements.

Summary

In this chapter, I ve presented a number of software testing techniques and strategies. You learned about the benefits of software testing and how to leverage test-driven development in your software projects. I also presented the testing facilities available to you for testing MySQL. I showed you the MySQL test and benchmark suites and introduced you to the profiling scripts for MySQL. The knowledge of these testing facilities will prepare you to ensure your modifications to the MySQL source code are of the highest quality possible. With this knowledge, you are now ready to begin creating extensions and enhancements of the MySQL system that will meet the same high-quality standards that MySQL AB adheres to.9 Now that you have this information, you can begin to design your solution and include testing early in your design phase. The next chapter, which begins the second part of this book, introduces you to the most important tool in a developer s toolbox: debugging!

9. Why else would they have created and made available to you testing, benchmarking, and profiling tools

and extend the MySQL system. It introduces you to how the MySQL code can be modified and explains how you can use the system as an embedded database system. 5 reviews debugging skills and techniques to help make development easy and less prone to failure. Several debugging techniques are presented, along with the pros and cons of each. 6 contains a tutorial on how to embed the MySQL system in enterprise applications. 7 examines the MySQL pluggable storage engine capability, complete with examples and projects that permit you to build a sample storage engine. 8 presents the most popular modification to the MySQL code. You ll learn how to modify SQL commands to add new parameters and functions, and how to add new SQL commands.

his chapter discusses one of the most powerful tools any developer can wield: debugging. Good debugging skills help ensure that your software projects are easy to develop and less prone to failure. I ll also explore the most common techniques for debugging the MySQL system. If you have already developed solid debugging skills, feel free to skim the following sections and move on to the section Debugging MySQL.