DB2 Multi-Row Fetch Techniques
By Tom Nonmacher
DB2 11.1, like its predecessors, provides several efficient methods to fetch data from your database. Among the most powerful of these is the multi-row fetch capability. This technique allows you to retrieve multiple rows of data with a single fetch operation, reducing the number of network round trips and resulting in significant performance gains for your applications. In this blog post, we'll explore this technique in detail, along with relevant examples.
The multi-row fetch feature is available in both the SQL and COBOL programming environments. To use it in SQL, you must declare a cursor and then use the FETCH NEXT ROWSET command to retrieve the data. The syntax for this command is quite straightforward. You simply specify the number of rows you want to fetch in the FOR n ROWS clause. Here is an example:
DECLARE CURSOR1 CURSOR FOR SELECT COL1, COL2 FROM TABLE1
OPEN CURSOR1
FETCH NEXT ROWSET FROM CURSOR1 FOR 5 ROWS INTO :HOST-VAR1, :HOST-VAR2
CLOSE CURSOR1
In this example, the FETCH NEXT ROWSET command retrieves five rows of data at a time. The data is then stored in the host variables HOST-VAR1 and HOST-VAR2. This technique can be incredibly useful when you need to retrieve large amounts of data, as it minimizes the number of network round trips and improves the overall efficiency of your database operations.
The multi-row fetch feature is not exclusive to DB2. Other database systems such as SQL Server 2016 and 2017, MySQL 5.7, and Azure SQL also offer similar functionality. In SQL Server, for example, you can use the OFFSET FETCH clause to retrieve a specific set of rows. Here is an example:
SELECT COL1, COL2 FROM TABLE1
ORDER BY COL1
OFFSET 10 ROWS
FETCH NEXT 5 ROWS ONLY
This SQL Server query orders the data by COL1, skips the first 10 rows, and then fetches the next 5 rows. It's a powerful technique that can be particularly useful when you need to implement pagination in your applications.
MySQL 5.7 also offers a similar feature. You can use the LIMIT clause to specify the number of rows to retrieve, and the OFFSET clause to specify the starting point. Here is an example:
SELECT COL1, COL2 FROM TABLE1
ORDER BY COL1
LIMIT 5 OFFSET 10
This MySQL query orders the data by COL1, skips the first 10 rows, and then fetches the next 5 rows. It's an efficient way to retrieve data in large databases.
In conclusion, the multi-row fetch feature is a powerful technique that can greatly improve the efficiency of your database operations. By reducing the number of network round trips, you can retrieve large amounts of data more quickly and efficiently. Whether you are working with DB2, SQL Server, MySQL, or Azure SQL, understanding how to use this feature can be a valuable asset in your database management toolkit.