Monday, March 16, 2015

SQL Performance Basics: Part 2, Rely on Indexes

Perhaps the single most important aspect of SQL tuning is indexing. All developers should know all of the indexes that exist on any table upon which they write SQL statements. When an index exists on a column (or columns), DB2 can use the index to reduce I/O instead of scanning the entire table to satisfy a predicate. 

For critical queries, where no useful indexes exists, it usually makes sense to create an index to improve query performance. Of course, developers should enlist the assistance of a DBA to ensure the creation of appropriate indexes.

Let's learn with Bsome basics. For example, consider this SQL statement:
    FROM     EMP 
    WHERE    EMPNO = '000010' 
    AND      DEPTNO =  'D01'
What index or indexes would make sense for this simple query? ""'First, think about all the possible indexes that you could create. Your first short list probably looks something like this:
  • Index1 on EMPNO
  • Index2 on DEPTNO
  • Index3 on EMPNO and DEPTNO
This is a good start, and Index3 is probably the best of the lot. It lets DB2 use the index to immediately look up the row or rows that satisfy the two simple predicates in the WHERE clause. Of course, if you already have a lot of indexes on the EMP table, you might want to examine the impact of creating yet another index on the table. There are several factors to consider. 

Indexing Factors to Consider
For starters, you need to weigh the impact of data modification. DB2 will automatically maintain every index you create. This means every INSERT and every DELETE to this table will insert and delete not just from the table, but also from its indexes. And if you UPDATE the value of a column that is in an index, you also update the index. So, indexes speed the process of retrieval but slow down modification.
You should also consider the impact to any existing indexes and applications before creating a new index. If an index already exists on EMPNO or DEPTNO, it might not be wise to create another index on the combination. However, it might make sense to change the other index to add the missing column. But not always, because the order of the columns in the index can make a big difference depending on the query. For example, consider this query:
WHERE    EMPNO = '000010' 
AND      DEPTNO >  'D01';
In this case, EMPNO should be listed first in the index. And DEPTNO should be listed second, allowing DB2 to do a direct index lookup on the first column (EMPNO) and then a scan on the second (DEPTNO) for the greater-than.
Furthermore, if indexes already exist for both columns (one for EMPNO and one for DEPTNO), DB2 can use them both to satisfy this query so creating another index might not be necessary.
Finally, you should consider the importance of the query you are attempting to tune. The more important the query, the more you might want to tune by index creation. Of course, the term "importance" is not always easy to quantify. If you are coding a query that the CEO will run every day, you want to make sure it delivers optimal performance. So building indexes for that particular query is important. On the other hand, a query for a clerk might not necessarily be weighted as high, so that query might have to make do with the indexes that already exist. Of course, the decision depends on the application's importance to the business--not just on the user's importance. If the clerk runs business critical transactions and the CEO is simply printing off a report for later consumption, then the importance varies... right?

Index Overloading
Index design involves much more than I have covered so far. For example, you might consider index overloading to achieve index-only access. If all the data that a SQL query asks for is contained in the index, DB2 might be able to satisfy the request using only the index. Consider our previous SQL statement. We asked for LASTNAME and SALARY, given information about EMPNO and DEPTNO. And we also started by creating an index on the EMPNO and DEPTNO columns. If we include LASTNAME and SALARY in the index as well, we never need to access the EMP table because all the data we need exists in the index. This technique can significantly improve performance because it cuts down on the number of I/O requests.
Keep in mind that making every query an index-only access is not prudent or even possible. You should save this technique for particularly troublesome or important SQL statements.

The Bottom Line
A solid understanding of the indexes that exist -- and how additional indexes can help -- is vital to the performance of your DB2 applications. Take the time to understand the indexes that exist for your applications and you can become a better developer, becoming more valuable to your organizations and earning the respect of your peers!

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