SQL Coding Guidelines: The Basics
When you are writing your SQL statements to access DB2 data be sure to follow the subsequent guidelines for coding SQL for performance. These are certain very simple, yet important rules to follow when writing your SQL statements. Of course, SQL performance is a complex topic and to understand every nuance of how SQL performs can take a lifetime. That said, adhering to the following simple rules puts you on the right track to achieving high-performing DB2 applications.
- DB2 tables may need to be changed in the future to include additional columns. SELECT * will retrieve those new columns, too, and your program may not be capable of handling the additional data without requiring time-consuming changes.
- DB2 will consume additional resources for every column that requested to be returned. If the program does not need the data, it should not ask for it. Even if the program needs every column, it is better to explicitly ask for each column by name in the SQL statement for clarity and to avoid the previous pitfall.
SELECT EMPNO, LASTNAME, SALARY
These rules, though, are not the be-all, end-all of SQL performance tuning – not by a long shot. Additional, in-depth tuning may be required. But following the above rules will ensure that you are not making “rookie” mistakes that can kill application performance.