Dynamic Performance Views
Dynamic Performance Views, also known as V$ views, are special views in Oracle Database that provide real-time information about the state and performance of the database. They are essential for performance tuning, troubleshooting, and monitoring. Unlike Data Dictionary Views, which provide static metadata about the database structure, Dynamic Performance Views give insights into the behavior and current state of the database.
Main Dynamic Performance Views
V$SESSION
Structure:
- SID: Session Identifier.
- SERIAL#: Session Serial Number.
- USERNAME: Name of the connected user.
- STATUS: Status of the session (ACTIVE, INACTIVE).
- OSUSER: Operating System user name.
- PROGRAM: Name of the program initiating the session.
Example Query:
-- List all active sessions SELECT sid, serial#, username, status, osuser, program FROM v$session WHERE status = 'ACTIVE';
Description:
- Provides information about user sessions, including details about their current activity and the programs they are using.
V$SQL
Structure:
- SQL_ID: Unique identifier for the SQL statement.
- SQL_TEXT: Full text of the SQL statement.
- PARSE_CALLS: Number of times the statement has been parsed.
- EXECUTIONS: Number of times the statement has been executed.
- SHARABLE_MEM: Memory shared by the SQL statement.
Example Query:
-- Find the most executed SQL statements SELECT sql_id, sql_text, executions, sharable_mem FROM v$sql ORDER BY executions DESC;
Description:
- Displays information about SQL statements currently executing or recently executed, useful for identifying resource-intensive queries.
V$SYSTEM_EVENT
Structure:
- EVENT: Name of the performance event.
- TOTAL_WAITS: Total number of waits for the event.
- TIME_WAITED: Total time spent waiting for the event.
- AVERAGE_WAIT: Average wait time for the event.
Example Query:
-- View the most frequent performance events SELECT event, total_waits, time_waited, average_wait FROM v$system_event ORDER BY total_waits DESC;
Description:
- Provides information on different types of performance events occurring in the system, helping to identify bottlenecks.
V$SESSION_WAIT
Structure:
- SID: Session Identifier.
- SERIAL#: Session Serial Number.
- EVENT: Event the session is waiting for.
- WAIT_TIME: Elapsed wait time.
- WAIT_TIME_MILLI: Elapsed wait time in milliseconds.
Example Query:
-- Find sessions with the longest wait times SELECT sid, serial#, event, wait_time, wait_time_milli FROM v$session_wait ORDER BY wait_time DESC;
Description:
- Displays details about sessions waiting on events, useful for diagnosing performance issues related to session waits.
V$SYSSTAT
Structure:
- NAME: Name of the system statistic.
- VALUE: Current value of the statistic.
Example Query:
-- View key system statistics
SELECT name, value
FROM v$sysstat
WHERE name IN ('parse count (total)', 'execute count');
Description:
- Provides important system statistics, such as the total number of parses or executions, useful for evaluating system workload.
V$DATABASE
Structure:
- DB_NAME: Name of the database.
- DB_BLOCK_SIZE: Size of the database blocks.
- CREATED: Date of database creation.
- RESETLOGS_CHANGE#: Change number after the last RESETLOGS.
Example Query:
-- Get information about the database SELECT db_name, db_block_size, created, resetlogs_change# FROM v$database;
Description:
- Provides general information about the database, useful for configuration checks and database management.
Practical Uses of Dynamic Performance Views
Dynamic Performance Views play a crucial role in performance analysis and troubleshooting. Here are some typical use cases:
- Performance Diagnostics: Use V$SQL to identify slow or resource-intensive queries. Combine this with V$SESSION to see which sessions are running these queries.
- Lock Management: Analyze V$SESSION_WAIT to understand session waits and identify locking or resource contention issues.
- Event Monitoring: Check V$SYSTEM_EVENT to see which performance events occur most frequently and where resources are commonly blocked.
- Statistics Analysis: Use V$SYSSTAT to monitor key statistics like the number of parses and executions, which helps in evaluating system performance.
Conclusion
Dynamic Performance Views are powerful tools for managing and tuning Oracle Database performance. They provide real-time insights and are essential for diagnosing issues, analyzing performance, and optimizing database operations. Understanding these views enables you to effectively monitor and troubleshoot database performance.