Oracle Database Statistics and Cost-Based Optimizer for Performance
Table of Contents
Introduction
Optimizing Oracle Database performance is essential for effective data management, and understanding Oracle database statistics is a key component of this process. The Cost-Based Optimizer (CBO) relies heavily on these statistics to determine the most efficient execution plans for SQL queries. In this guide, we'll delve into the significance of Oracle database statistics, how to gather them, and the best practices for managing them to achieve optimal performance.
What is the Cost-Based Optimizer?
The Cost-Based Optimizer (CBO) is a crucial feature of the Oracle Database that evaluates different execution plans for SQL queries. It uses Oracle database statistics to calculate the "cost" of each plan based on factors like I/O, CPU usage, and network resources. By selecting the plan with the lowest cost, the CBO ensures that queries are executed in the most efficient manner, reducing resource consumption and enhancing overall database performance.
Importance of Oracle Database Statistics
Oracle database statistics are vital for the CBO to make informed decisions about query execution plans. These statistics include data on distribution, table sizes, index usage, and more. Accurate and current Oracle database statistics allow the CBO to generate optimal execution plans, which improves query performance and resource efficiency. Without updated statistics, the CBO may choose less effective plans, negatively affecting database performance.
Gathering Oracle Database Statistics
There are several methods for gathering Oracle database statistics. Here’s an overview of the key procedures:
Automatic Statistics Collection
Oracle automatically collects statistics during maintenance windows using the DBMS_STATS package. This ensures that Oracle database statistics are regularly updated without manual intervention. You can verify the settings and status of automatic statistics collection with the following query:
SELECT * FROM dba_scheduler_windows;
Manual Statistics Gathering
Manual gathering of Oracle database statistics might be necessary after major data changes or for tuning specific queries. Use the GATHER_TABLE_STATS procedure to collect statistics for a specific table:
Table Statistics
To gather statistics for a specific table:
BEGIN
DBMS_STATS.GATHER_TABLE_STATS(
ownname => 'DBDOCS',
tabname => 'EMPLOYEES',
cascade => TRUE,
estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE,
degree => DBMS_STATS.AUTO_DEGREE
);
END;
/
Schema Statistics
To gather statistics for all tables within a schema, use:
BEGIN
DBMS_STATS.GATHER_SCHEMA_STATS(
ownname => 'DBDOCS',
cascade => TRUE,
estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE,
degree => DBMS_STATS.AUTO_DEGREE
);
END;
/
Database Statistics
For comprehensive statistics collection across the entire database, execute:
BEGIN
DBMS_STATS.GATHER_DATABASE_STATS(
cascade => TRUE,
estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE,
degree => DBMS_STATS.AUTO_DEGREE
);
END;
/
Managing Oracle Database Statistics
Proper management of Oracle database statistics ensures continued optimal performance. Here are some essential management tasks:
Locking Statistics
To prevent changes to statistics, lock them using:
BEGIN
DBMS_STATS.LOCK_TABLE_STATS(
ownname => 'DBDOCS',
tabname => 'EMPLOYEES'
);
END;
Restoring Statistics
Restore previous Oracle database statistics if necessary with:
BEGIN
DBMS_STATS.RESTORE_TABLE_STATS(
ownname => 'DBDOCS',
tabname => 'EMPLOYEES',
as_of_timestamp => SYSTIMESTAMP - INTERVAL '1' DAY
);
END;
Deleting Statistics
To remove outdated statistics, use:
BEGIN
DBMS_STATS.DELETE_TABLE_STATS(
ownname => 'DBDOCS',
tabname => 'EMPLOYEES'
);
END;
Conclusion
Effectively managing Oracle database statistics is essential for optimizing your database performance. Regular collection and strategic management of Oracle database statistics allow the Cost-Based Optimizer to make well-informed decisions, leading to improved query performance and efficient resource use. By adhering to these best practices, you can ensure that your Oracle database performs at its best.
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