Understanding EXPLAIN PLAN【每日一译】--2013-1-19
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optimizer for SELECT, UPDATE, INSERT, and DELETE statements. A statement's
execution plan is the sequence of operations Oracle performs to run the statement.
The row source tree is the core of the execution plan. It shows the following
information:
EXPLAIN PLAN语句显示执行计划,它被O优化器选择用于SELECT,UPDATE,INSERT
和DELETE语句。一个语句的执行计划是O执行运行语句的操作顺序。行资源树是执行
计划的核心。它显示了如下信息:
■ An access method for each table mentioned in the statement
■ A join method for tables affected by join operations in the statement
■ Data operations like filter, sort, or aggregation
#语句相关表的访问顺序
#语句中涉及到的每个表的访问方法
#语句中有关联的表的连接方法
#数据操作比如过滤,排序,或聚合
following:
■ Optimization, such as the cost and cardinality of each operation
■ Partitioning, such as the set of accessed partitions
■ Parallel execution, such as the distribution method of join inputs
The EXPLAIN PLAN results let you determine whether the optimizer selects a
particular execution plan, such as, nested loops join. It also helps you to understand
the optimizer decisions, such as why the optimizer chose a nested loops join instead of
a hash join, and lets you understand the performance of a query
另外对于行资源树,执行计划表包含信息如下:每个操作的成本和基数
#分区,比如访问的分区集
#并行执行,比如合并输入分配方法
EXPLAIN PLAN结果让你决定优化器是否选择一个指定的执行计划,比如嵌套循环连接,它也
可以帮你去理解优化器的决定,比如为什么优化器选择嵌套循环连接取代HASH连接,并且让
你理解查询的性能。
#优化器,比如
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