explain usage in oracle

EXPLAIN is used in Oracle to analyze the execution plan of a SQL statement to help the optimizer choose the best path. By adding EXPLAIN PLAN FOR before the SELECT statement, you can obtain a text report containing information about the execution path, operation type, cost, predicates, etc. When analyzing a plan, paths are identified, costs are checked, predicates are analyzed, indexes are considered, and actions are taken based on the output to optimize performance, such as creating indexes or rewriting queries.

Usage of EXPLAIN in Oracle

EXPLAIN is a useful tool in Oracle that can be used to analyze SQL statements execution plan. It helps the database optimizer choose the best execution path by providing detailed reports on how statements executed.

How to use EXPLAIN

To use EXPLAIN, add it as a prefix before the SELECT statement, as follows:

<code>EXPLAIN PLAN FOR <SQL 语句>;</code>

For example:

<code>EXPLAIN PLAN FOR SELECT * FROM employees WHERE department_id = 10;</code>

EXPLAIN Output

EXPLAIN output is a text report containing the following information:

• ID: Execution Plan The unique identifier of the step in the .
• Operation: The type of operation being performed (e.g. TABLE ACCESS, INDEX RANGE SCAN).
• Options: Any options associated with the operation (e.g. INDEX_RANGE_SCAN(STARTKEY, STOPKEY)).
• Rows: Estimate the number of rows returned by this operation.
• Cost: Estimated execution cost of the operation.
• Parent: The ID of the parent operation.
• Predicate Information: Any predicate information used for optimization operations.

Using EXPLAIN to analyze the execution plan

To analyze the execution plan, follow these steps:

1. Identification Execution path: EXPLAIN The first line in the output represents the root operation of the statement. From here, follow the Parent column to understand the execution path.
2. Check Cost: The Cost column indicates the estimated cost of each operation. A higher cost means the operation is more expensive.
3. Analyze predicate information: The Predicate Information column displays the any predicate used to filter rows. Make sure the predicate is correct and selective.
4. Consider indexes: EXPLAIN output will show the operations used to access the table and index. If the index is not being used, check the definition of the index and the distribution of data in the table.
5. Optimization operations: Based on the EXPLAIN output, you can take steps to optimize the operation, such as creating indexes, adjusting predicates, or rewriting queries.

Using EXPLAIN, you can gain insight into how Oracle executes SQL statements and take steps to optimize its performance.

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