14. Using the Query Optimizer : Query Execution Plans
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Query Execution Plans
When the query optimizer evaluates a query (such as the SQL statements SELECT, INSERT, UPDATE, DELETE, and CREATE TABLE...AS), it generates a QEP showing how the query is executed. Once the QEP has been generated, it can be used one or more times to execute the same query. Because there are often many different ways to optimize a given query, choosing the best QEP can have a significant impact on performance.
You can display a diagram or graph of the QEP selected, which can be used to gain insight into how queries are handled by the query optimizer. Knowing how to read and analyze QEPs can allow you to detect, and often avoid, hidden performance problems. After examining a QEP you can, for example, decide that you need to optimize your database to provide the optimizer with better statistics, as described in Database Statistics.
Examining QEPs can help you understand what is involved in executing complex queries in single-user situations.