oracle explain怎么看

Oracle的`EXPLAIN PLAN`，可以用于分析SQL查询语句的执行计划，以帮助我们优化查询性能。

1. 执行`EXPLAIN PLAN`语句

在查询语句前加上`EXPLAIN PLAN FOR`，例如：

EXPLAIN PLAN FOR SELECT * FROM employees WHERE department_id = 10;

2. 查看执行计划

执行完`EXPLAIN PLAN`语句后，可以通过`DBMS_XPLAN`包中的`DISPLAY`过程来查看执行计划，例如：

SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

该语句将会输出查询的执行计划，包括操作、表名、访问方法、行数等信息。我们可以根据这些信息来判断查询语句是否存在性能问题，并作出相应的优化措施。

需要注意的是，`EXPLAIN PLAN`得到的执行计划是一个估算值，实际执行的情况可能会因为数据量、索引状态等因素而有所不同。

相关问题

oracle explain

Oracle Explain Plan is a tool used for analyzing and understanding how Oracle will execute a SQL statement. It provides information on the operations that will be performed by Oracle to fetch the requested data, the order in which the operations will be executed, and the expected cost of each operation.

The Explain Plan is generated by the Oracle Optimizer, which analyzes the SQL statement and determines the most efficient way to execute it. The optimizer considers factors such as the available indexes, the size of the tables, and the complexity of the query.

The Explain Plan can be obtained using the EXPLAIN PLAN statement or by using tools such as SQL Developer or Toad. The output of the Explain Plan includes the execution plan, the estimated number of rows, and the cost of each operation.

The execution plan provides a visual representation of the operations that Oracle will perform to execute the SQL statement. The estimated number of rows indicates the number of rows that Oracle expects to retrieve for each operation. The cost of each operation is an estimate of the resources required to execute the operation, including disk I/O, CPU time, and memory usage.

The Explain Plan is a valuable tool for optimizing SQL statements and improving the performance of Oracle databases. By analyzing the plan, database administrators and developers can identify potential bottlenecks and make changes to the SQL statement or database schema to improve performance.

oracle explain执行计划

Oracle的Explain执行计划是一个用于评估查询性能的工具。当你执行一个查询语句时，Oracle会生成一个查询执行计划，它描述了Oracle数据库引擎如何处理查询，并指导数据库选择最佳执行路径。

你可以使用EXPLAIN PLAN语句来获取查询执行计划。例如，假设你有一个查询语句如下：

SELECT *
FROM employees
WHERE department_id = 100;

要获取该查询的执行计划，你可以将EXPLAIN PLAN语句与查询语句结合使用，如下所示：

EXPLAIN PLAN FOR
SELECT *
FROM employees
WHERE department_id = 100;

执行以上语句后，你可以使用以下语句查看执行计划：

SELECT *
FROM TABLE(DBMS_XPLAN.DISPLAY);

这将显示查询的执行计划信息，包括访问方法、连接顺序、索引使用情况等。通过分析执行计划，你可以了解查询的性能瓶颈，并根据需要进行优化。

需要注意的是，执行计划是一种近似估算，实际执行中可能会有一些微小的差异。因此，在优化查询性能时，最好使用实际的执行计划和性能统计信息进行分析。