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In the world of database management, achieving optimal performance is a constant pursuit. One of the critical aspects of this quest is leveraging indexes to their full potential. Indexes, when used correctly, can significantly speed up data retrieval operations, and DB2 11.5 provides an excellent feature known as "Index Only Access" to optimize the performance. This blog post aims to shed some light on how to tune the performance of Index Only Access in DB2.

Index Only Access is a feature where DB2 uses the index to satisfy a query without actually having to access the underlying table data. This method can drastically reduce the disk I/O operations, subsequently leading to a significant performance boost. However, for DB2 to use Index Only Access, the index must include all the columns referenced in the query, including those in the SELECT, WHERE, ORDER BY, and GROUP BY clauses.

-- DB2 Index Only Access Example
CREATE INDEX idx_orders ON orders (order_id, customer_id, order_date);
SELECT order_id, customer_id FROM orders WHERE order_date = '2021-06-01';

In the above example, the query would use Index Only Access because the index includes all the columns used in the SELECT and WHERE clauses. However, if the query included a column that isn't in the index, DB2 would have to access the table data, negating the performance benefits of Index Only Access.

The methods of tuning Index Only Access in DB2 can be broadly classified into two categories: creating covering indexes and reducing index page splits. A covering index is an index that includes all the columns required by a query. If a query's columns are all included in an index, DB2 can retrieve the query data without having to access the table data, resulting in faster retrieval times. On the other hand, reducing index page splits involves maintaining a low fill factor for your indexes, which leaves space for additional index entries and reduces the need for page splits when new data is inserted.

-- DB2 Covering Index Example
CREATE INDEX idx_orders_covering ON orders (order_id, customer_id, order_date, total_amount);
SELECT order_id, customer_id, total_amount FROM orders WHERE order_date = '2021-06-01';

In the above example, the index 'idx_orders_covering' is a covering index for the query because it includes all the columns used in the query. DB2 can use this index to satisfy the query without having to access the table data, leading to a performance boost.

While tuning DB2 Index Only Access is important, it's equally crucial to note that these principles can be applied to other database management systems such as SQL Server 2019, MySQL 8.0, Azure SQL, and Azure Synapse. Each of these platforms offers its unique features and capabilities for index tuning and management, and understanding these can greatly enhance the performance of your database applications.