What are some common pitfalls to avoid when designing DB2 indexes?

1 Choosing the wrong columns

One of the most important decisions when designing an index is which columns to include and in what order. The columns you choose should match the columns you use in your predicates, join conditions, and order by clauses. Otherwise, the index might not be used at all, or only partially. You should also avoid indexing columns that have low cardinality, meaning they have few distinct values, such as gender or status. These columns are not very selective and can lead to inefficient scans or index-only access. Instead, focus on columns that have high cardinality and can narrow down the search space quickly.

2 Creating too many or too few indexes

Another common pitfall is creating too many or too few indexes for your tables. Having too many indexes can slow down your insert, update, and delete operations, as well as consume more disk space and memory. Having too few indexes can cause your queries to run slower, as they have to scan more rows or pages. The optimal number of indexes depends on several factors, such as the size and structure of your tables, the frequency and complexity of your queries, and the workload and performance requirements of your system. You should monitor and analyze your queries and indexes regularly, and use tools such as the Explain function or the Index Advisor to identify potential problems and opportunities.

3 Ignoring the clustering factor

The clustering factor is a measure of how well the rows in a table are physically ordered according to the index key. A high clustering factor means that the rows are well aligned with the index, which reduces the number of page reads and improves the query performance. A low clustering factor means that the rows are scattered across the table, which increases the number of page reads and degrades the query performance. You can improve the clustering factor by creating a clustered index on the most frequently used column or columns, or by reorganizing your table periodically using the REORG command.

4 Neglecting the index maintenance

Creating an index is not a one-time task. You need to maintain your indexes regularly to ensure they remain up to date, accurate, and efficient. The RUNSTATS command can be used to collect and update the statistics on your tables and indexes, which is necessary for the optimizer to choose the best access plan for your queries. Additionally, the REBUILD or REORG commands can improve the index structure, reduce fragmentation, and reclaim unused space. Lastly, the DROP or ALTER commands can be used to remove unnecessary or obsolete indexes, or change their properties or options.

5 Forgetting the index design guidelines

When designing DB2 indexes, it is important to follow the best practices and guidelines. These guidelines include using short and simple index keys to reduce the index size and improve performance, compressing indexes to save disk space, partitioning tables and indexes to improve scalability and manageability, and using index expressions, predicates, and functions to enhance functionality. By avoiding common pitfalls, you can create DB2 indexes that optimize your queries and improve system performance.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?