Understanding EXPLAIN PLAN【每日一译】--2013-1-19

The EXPLAIN PLAN statement displays execution plans chosen by the Oracle
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 ordering of the tables referenced by the statement
■ 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

In addition to the row source tree, the plan table contains information about the

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