Oracle’s DBMS_XPLAN package is a utility designed to provide detailed and organized insights into the execution plans of SQL queries. It is a fundamental tool for database administrators (DBAs) and developers alike, offering deep visibility into how the Oracle Database executes SQL statements. Understanding and utilizing this package is critical for optimizing queries, managing system performance, and ensuring that the database operates efficiently.
1. Introduction to Execution Plans and the DBMS_XPLAN Package
Before delving into DBMS_XPLAN, it’s essential to understand the concept of an execution plan. When an SQL query is issued, the Oracle Database determines the most efficient way to execute it. The execution plan describes the exact operations Oracle will perform to retrieve or modify the data as requested.
An execution plan is crucial for:
- Identifying inefficient queries
- Tuning SQL statements
- Understanding how Oracle processes queries (e.g., join methods, access paths, and the use of indexes)
The DBMS_XPLAN package is Oracle’s built-in utility for displaying these execution plans. It complements the EXPLAIN PLAN statement, which is commonly used to generate execution plans. However, EXPLAIN PLAN only predicts the execution plan without necessarily reflecting the actual runtime behavior, whereas DBMS_XPLAN can show both predicted and actual performance depending on the query type.
2. How to Use the DBMS_XPLAN Package
The DBMS_XPLAN package consists of several procedures and functions that extract and format execution plans from Oracle’s internal structures, typically from either:
The PLAN_TABLE, a temporary table where the output of the EXPLAIN PLAN command is stored.
The V$SQL_PLAN view, which contains actual execution plans for queries that have been executed.
Before generating an execution plan, ensure the following:
You have permission to query the PLAN_TABLE or V$SQL_PLAN
The query you are analyzing is either still available in the shared SQL area (for real-time plans) or its plan has been previously captured (for historical analysis)
The typical workflow involves:
Using EXPLAIN PLAN to generate a predicted plan:
EXPLAIN PLAN FOR SELECT * FROM employees WHERE employee_id = 101;
Viewing the execution plan with DBMS_XPLAN:
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);
Oracle automatically populates the PLAN_TABLE after executing EXPLAIN PLAN, and the DBMS_XPLAN package formats its content for easy readability.
Alternatively, if you want to display the actual execution plan for a running or previously executed SQL statement:
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(sql_id => 'sql_id_value', format => 'ALLSTATS LAST'));
This retrieves the plan from the V$SQL_PLAN view, along with runtime statistics (like row counts and memory usage) for the query.
3. Different Display Functions in DBMS_XPLAN
The DBMS_XPLAN package contains several key functions for displaying execution plans. Each function is tailored to specific use cases:
DISPLAY
Displays the contents of the PLAN_TABLE.
Syntax:
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY([table_name], [statement_id], [format]));
Example:
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);
DISPLAY_CURSOR
Displays the actual execution plan from memory (V$SQL_PLAN) for SQL statements in the shared SQL area.
Syntax:
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR([sql_id], [child_number], [format]));
Example:
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(sql_id => 'abcd1234', format => 'ALLSTATS LAST'));
DISPLAY_AWR
Displays an execution plan from historical data stored in the Automatic Workload Repository (AWR).
Syntax:
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_AWR([sql_id], [plan_hash_value], [format]));
Example:
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_AWR(sql_id => 'abcd1234', plan_hash_value => 1234567890));
DISPLAY_SQLSET
Retrieves the execution plan for a specific SQL statement stored in a SQL tuning set (STS).
Syntax:
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_SQLSET([sqlset_name], [sql_id], [format]));
Example:
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_SQLSET(sqlset_name => 'my_sql_set', sql_id => 'abcd1234'));
4. Understanding Execution Plan Output
Once you’ve generated an execution plan, it’s critical to understand its components to make effective use of the information. Execution plans are typically displayed in a tabular format, with each row representing an operation (or “step”) performed by Oracle to execute the SQL query.
Key columns in the output include:
ID: The step number, indicating the order in which Oracle performs operations.
Operation: Describes the type of operation, such as TABLE ACCESS, INDEX SCAN, or SORT.
Options: Additional details about the operation (e.g., whether the table access is by rowid or full scan).
Object Name: The name of the table, index, or other object involved in the operation.
Rows: An estimate of the number of rows processed by the step.
Cost: The relative cost of the operation, representing resource usage.
Cardinality: Predicted number of rows returned by the step.
Time: Estimated time taken for the step.
Here is an example of what an execution plan might look like:
-------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 12 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS FULL | EMPLOYEES | 1 | 12 | 2 (0)| 00:00:01 | --------------------------------------------------------------------------------
5. Key Sections of DBMS_XPLAN Output
Execution plans can contain a lot of detailed information. The format parameter passed to DBMS_XPLAN functions controls which sections and how much detail to display. Some key sections include:
Predicate Information: Displays the exact filters and conditions applied to rows in specific operations.
Key statistics: Shows actual row counts, memory consumption, and other statistics (when running with ALLSTATS in DISPLAY_CURSOR).
Remote Execution Steps: Shows information about distributed queries.
Parallel Execution: Displays details on how queries were parallelized (if applicable).
Different format options are available, including:
BASIC: Shows a minimal execution plan.
TYPICAL: Displays a moderate amount of information (default).
ALLSTATS: Includes runtime statistics like actual row counts and memory usage.
ADVANCED: Provides additional detailed diagnostic information useful for complex queries.
6. Optimizing Queries Using DBMS_XPLAN
Once an execution plan is retrieved, you can identify opportunities for optimization by analyzing its key operations:
Full Table Scans: If a table scan is reported, you might consider adding an index, especially if the scan affects a large table.
Index Usage: Check whether indexes are being utilized effectively. If not, you may need to tune index selection or query predicates.
Join Methods: Pay attention to join operations like nested loops, hash joins, or merge joins, and whether they are appropriate for the data size and query complexity.
Partitioning: If your tables are partitioned, ensure Oracle is using partition pruning when appropriate to minimize data scanning.
For instance, if an execution plan shows a TABLE ACCESS FULL operation on a large table, this might indicate that the query could benefit from indexing the relevant columns, or adjusting statistics to guide the optimizer towards using an existing index.
7. Advanced Topics in DBMS_XPLAN
7.1 Real-Time SQL Monitoring
For long-running queries, real-time SQL monitoring can provide dynamic insights into the performance of SQL statements as they execute. The DBMS_XPLAN package can display these real-time statistics using the DISPLAY_CURSOR function with the ALLSTATS LAST format.
7.2 Adaptive Execution Plans
Oracle’s adaptive query optimization features allow the database to adjust execution plans on the fly based on real-time statistics. The DBMS_XPLAN package can show when and how Oracle adapts execution plans.
7.3 Parallel Query Execution
If a query is being executed in parallel, the DBMS_XPLAN output can display the parallelization details, including how work is distributed across query slaves.
8. Best Practices and Common Mistakes
8.1 Best Practices:
Regularly monitor execution plans for long-running or frequently executed queries.
Use actual execution plans (DISPLAY_CURSOR) instead of predicted plans (EXPLAIN PLAN) when possible to get accurate runtime behavior.
Focus on high-cost operations such as full table scans, large sorts, or nested loop joins on large data sets.
Adjust optimizer statistics and use appropriate indexes based on plan output.
8.2 Common Mistakes:
Relying solely on EXPLAIN PLAN: This only predicts the plan and doesn’t reflect actual performance.
Ignoring optimizer statistics: Poor or outdated statistics can lead to suboptimal execution plans.
Over-indexing: Adding too many indexes can increase the cost of DML operations (insert, update, delete) and can sometimes lead the optimizer to choose inefficient plans.
9. Conclusion
The Oracle DBMS_XPLAN package is an essential tool for database administrators and developers aiming to optimize query performance. By providing detailed insights into SQL execution plans, DBMS_XPLAN helps users understand how the Oracle Database processes queries, identify performance bottlenecks, and implement optimizations that improve system efficiency.
Through functions like DISPLAY, DISPLAY_CURSOR, and DISPLAY_AWR, the package offers flexible options for retrieving both predicted and actual execution plans, enabling users to fine-tune queries based on real-world performance data. By mastering DBMS_XPLAN, database professionals can proactively manage query performance and ensure that their applications run at optimal speed.
Whether you’re debugging a slow query or proactively tuning a database, DBMS_XPLAN is a powerful ally in the ongoing quest for efficient, high-performance Oracle systems.