The IBM System i family, which encompasses the IBM AS/400, eServer iSeries, eServer i5, and System i5, has a successful 24-year history of satisfying hundreds of customers' requirements, thanks to its developers. DB2 for i5/OS, also known as DB2 Universal Database for iSeries, is deeply rooted in the architecture and heritage of the AS/400 and its predecessor System/38.

The database has undergone significant changes over the years to maintain its competitive advantage including fundamental changes to the structure of the database to compete successfully in the industry. In doing so, IBM Rochester launched a project to re-engineer and redesign important components of the database. The redesigned components were architected using object-oriented design concepts and then implemented using object-oriented implementation techniques.

The query optimizer was one of the key components that was redesigned. This IBM Redbooks publication gives a broad understanding of the architectural changes of the database concerning the query optimizer. It explains the following concepts:

• The architecture of the query optimizer
  

• The data access methods used by the query optimizer
  

• The Statistics Manager included in V5R2
  

• The feedback message changes of the query optimizer
  

• Some performance measurements
  

• Changes and enhancements made in V5R4
  

The objective of this book is to help you minimize any SQL or database performance issues when upgrading to OS/400 V5R2 or V5R3 or to IBM i5/OS V5R4. Prior to reading this book, you should have some knowledge of database performance and query optimization.

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