March 2012
Intermediate to advanced
442 pages
9h 31m
English
Content preview from IBM DB2 9.7 Advanced Application Developer CookbookBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Using explain utilities
An SQL query can have more than one possible way or path of execution (also known as access plan). Access plan is also known as explain plan, and these two terms are used interchangeably. The DB2 optimizer uses a cost-based optimization to choose the path with the most efficient cost. The cost of each plan is determined from the latest database statistics available. That's why it is very important to make sure that the system statistics are up-to-date. Access plans provide the following information:
- Decision criteria that were used to choose the access plan
- Sequence of operations that were performed to process the query
- Cost of each step performed
- Selectivity estimate for each predicate
- Values of special registers and other ...
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