By collecting statistics on tables, indexes, and other database objects, Oracle users can better understand core data distribution and usage patterns, improving performance and reducing resource consumption. Whether you’re a database administrator or a developer, understanding the Oracle stats-gathering process is key to maintaining a high-performing and reliable database environment.
This article explores the importance of effective stats gathering, the tools and methods Oracle provides for collecting statistics, and best practices to optimize overall database performance.
Key Takeaways
- Regular gathering of statistics is essential for optimizing query performance and maintaining database health in and across Oracle databases.
- Utilizing the DBMS_STATS package enables concurrent, customizable statistics gathering, significantly improving efficiency and minimizing downtime.
- Configuring automatic and high-frequency statistics collection is crucial for ensuring the optimizer has accurate and up-to-date information for effective query execution planning.
Understanding the Importance of Stats Gathering
Gathering statistics optimizes query performance in Oracle databases. Regularly collecting these statistics helps maintain database health. Accurate statistics allow the query optimizer to make informed decisions about execution plans, which is crucial for cost-based optimization.
Statistics can become stale with database changes, impacting performance. When table sizes and data distributions frequently change, statistics must be regenerated regularly. Statistics should also be updated after modifying a schema object’s data or structure. Periodic gathering ensures accuracy in query execution.
The optimizer assumes uniform data distribution, making accurate statistics crucial. Gathering statistics for fixed objects prevents suboptimal execution plans caused by default values. When updating statistics, consider the impact on optimizer decisions and performance consistency.
Changes in data volumes or schema objects should trigger the gathering of new statistics. Consistent execution plans rely on accurate statistics, which can be transferred between servers. Maintaining current fixed object statistics avoids suboptimal execution plans from non-representative default values.
Gathering Schema Statistics with DBMS_STATS
The DBMS_STATS package is a powerful tool for manipulating optimizer statistics in Oracle databases, offering advantages like parallel gathering and tailored options over the older ANALYZE command. Use the DBMS_STATS.GATHER_SCHEMA_STATS procedure to collect new statistics for the entire schema.
DBMS_STATS supports concurrent statistics gathering and is unaffected by parallel query processing. This flexibility is crucial for minimizing downtime and maximizing efficiency. The package allows for the modification of gathering options based on specific schema stats needs, enabling customization of the statistics collection process.
Concurrent statistics gathering can significantly reduce collection time, especially in large schemas. This ensures that the optimizer collects statistics using accurate and up-to-date information to enable concurrent statistics gathering for execution plans.
Full Syntax of GATHER_SCHEMA_STATS
The GATHER_SCHEMA_STATS procedure collects statistics for all objects in a specified schema, allowing various parameters to tailor the gathering process. For example, you can specify a target schema by using the OWNNAME parameter.
Understanding and utilizing the full syntax of GATHER_SCHEMA_STATS ensures comprehensive and accurate content-gathering schema stats. This practice enhances query performance and allows you to query the data dictionary, maintaining the overall health and efficiency of your Oracle database.
Collecting Table Statistics in Oracle
Collecting table statistics is crucial for optimal database performance, and users can leverage the DBMS_STATS.GATHER_TABLE_STATS command to do so. This command gathers statistics specific to a single table, ensuring the optimizer has accurate data for generating execution plans.
DBMS_STATS can collect individual statistics for each partition and global statistics for the entire table when dealing with partitioned tables. This dual-level approach accurately represents data distribution and ensures efficient query execution. For frequently modified tables, gather statistics on a weekly or monthly basis.
The cascade option in GATHER_TABLE_STATS also gathers statistics for indexes related to the table. This approach ensures all related objects have up-to-date statistics, enhancing query performance and database efficiency.
Full Syntax of GATHER_TABLE_STATS
The GATHER_TABLE_STATS procedure gathers statistics for a specified schema and table. The ESTIMATE_PERCENT parameter controls the percentage of rows sampled with DBMS_STATS. The recommended setting is AUTO_SAMPLE_SIZE. This method ensures an optimal balance between accuracy and performance.
The AUTO_SAMPLE_SIZE constant is the preferred sample size method for gathering statistics in Oracle. Verify table statistics availability by querying the DBA_TABLES view. The SAMPLE_SIZE column in DBA_TABLES shows the actual sample size used for gathering statistics. This syntax ensures you can gather accurate and representative statistics for your tables.
Gathering Index Statistics Efficiently
Index statistics are crucial for query optimization, providing the optimizer with information about the distribution and usage of index entries. The GATHER_INDEX_STATS command gathers index stats, ensuring the optimizer has accurate data for efficient execution plans.
The cascade parameter in GATHER_SCHEMA_STATS specifies whether to gather statistics for dependent objects like indexes. The COMPUTE STATISTICS option automatically gathers statistics when creating or rebuilding an index. Oracle gathers index statistics during the creation or rebuilding of a B-tree or bitmap index when using the COMPUTE STATISTICS option.
Sampling can be used for parallel statistics gathering, but cluster indexes, domain indexes, and bitmap join indexes cannot be gathered in parallel. If not using the COMPUTE STATISTICS clause, use the DBMS_STATS.GATHER_INDEX_STATS procedure.
Full Syntax of GATHER_INDEX_STATS
The AUTO_CASCADE parameter specifies whether to gather statistics for all indexes in Oracle 10g and upwards. Specify the number of buckets in the METHOD_OPT argument when collecting histograms for index expressions. GATHER_INDEX_STATS gathers statistics for one or more indexes in an Oracle database to improve performance and optimization.
These parameters optimize statistics gathering for indexes, significantly influencing associated query performance. Understanding the full syntax of GATHER_INDEX_STATS ensures your index statistics are accurate and up-to-date.
Configuring Automatic Optimizer Statistics Collection
Automatic optimizer statistics collection ensures the optimizer has accurate, up-to-date information for generating execution plans. By default, this process runs daily during scheduled maintenance windows. Configuring automatic optimizer statistics ensures stale statistics are refreshed regularly, maintaining optimal query performance.
Oracle recommends against disabling automatic statistics gathering, as it’s crucial for generating efficient query execution plans. High-frequency automatic statistics collection, set to run every 15 minutes, reduces the chances of stale statistics, ensuring the optimizer always has current data for efficient query execution.
Automatic statistics gathering is critical for the optimizer to generate optimal query plans. Configuring it ensures your database remains performant and efficient.
High-Frequency Automatic Optimizer Statistics Collection
High-frequency automatic optimizer statistics collection gathers statistics more frequently to reduce staleness. To configure high-frequency statistics collection, use the DBMS_STATS.SET_GLOBAL_PREFS procedure to define preferences like execution interval and maximum runtime. This procedure fine-tunes the frequency and duration of statistics collection tasks.
Enable the Resource Manager to manage resources effectively during concurrent statistics gathering. High-frequency statistics collection can be set to a minimum of 60 seconds, but balancing accurate statistics provision against processing overhead is important.
Configuring high-frequency automatic optimizer statistics collection ensures your database always has accurate and up-to-date statistics.
Gathering System Statistics Manually
Gathering system statistics is crucial for optimizing Oracle database performance, especially for evaluating I/O and CPU costs. System statistics describe hardware characteristics like I/O and CPU performance, providing necessary insight for efficient query planning. The use of DBMS_STATS.GATHER_SYSTEM_STATS enables users to gather system statistics and collect performance data under different conditions.
Delete existing system statistics by executing DBMS_STATS.DELETE_SYSTEM_STATS and restarting the database to reset values. This process ensures outdated or inaccurate system statistics don’t affect query performance.
Gathering system statistics manually ensures the optimizer has accurate information about your hardware environment and allows you to gather and manage optimizer statistics manually.
Running Statistics Gathering Functions in Reporting Mode
DBMS_STATS can execute statistics gathering in reporting mode, identifying objects needing statistics without actually collecting them. Running statistics gathering in reporting mode doesn’t alter existing statistics, preserving the current state of the database object.
Using reporting mode helps make informed decisions about when and where to gather statistics, ensuring database efficiency and performance across organizational units.
Checking and Managing Stale Statistics
Stale statistics can significantly impact Oracle database performance. Monitoring and managing these statistics is crucial for maintaining optimal performance. Use the DBA_TAB_STATISTICS view to check for stale statistics on a table. The reporting mode of DBMS_STATS helps assess the status of statistics for specific database objects.
Monitor stale statistics by checking the STALE_STATS column in the DBA_TAB_STATISTICS view. Frequent data changes between collection tasks can cause performance problems due to stale statistics. DBMS_STATS determines if statistics are stale based on whether a monitored table has been modified. A table is considered stale when a predefined percentage of its total rows have been modified, indicating a significant change in the data.
View statistics related to indexes by querying the DBA_IND_STATISTICS view. Regularly checking and managing stale statistics ensures your database remains efficient and performant.
Refreshing Stale Stats Automatically
Refreshing stale statistics is crucial for maintaining optimal database performance. Locking statistics in Oracle prevents changes made by automated jobs, ensuring consistency in the statistics used for query performance optimization.
Automated maintenance tasks and predefined windows help refresh stale statistics regularly, ensuring that your database always has the most accurate and up-to-date information.
Using Histograms for Data Distribution
Histograms enhance the optimizer’s ability to make informed decisions by providing accurate estimates of column data distribution.
Frequency histograms are generated when the count of distinct values is the same as or lower than the number of buckets. This ensures proper representation of the data within the defined intervals. Height-balanced histograms come into play when the number of distinct values exceeds the number of histogram buckets, ensuring a more uniform distribution.
Value-based histograms are formed when the distinct values do not exceed the number of histogram buckets, ensuring each bucket represents a unique value. A height-based histogram organizes its values into bands with approximately equal numbers of values. Histograms can be created using the GATHER_SCHEMA_STATS procedure, with the method_opt parameter controlling their creation.
For columns with highly skewed data distribution, creating accurate histograms is crucial. This is particularly relevant for those columns that are frequently used in WHERE clauses. If the data distribution of a column changes frequently, a histogram must be recomputed often. This is necessary to ensure an accurate representation of the data.
By using histograms, you can ensure that the optimizer has the most accurate information about data distribution, leading to better query performance.
Creating and Verifying Histograms
Creating histograms is a crucial step in representing data distribution accurately. A histogram typically has a default of 75 buckets. This serves as the standard configuration for data representation. When the number of distinct values in a column is small, you should set the number of buckets greater than the number of distinct values.
When creating histograms, the SIZE keyword defines the maximum number of histogram buckets. Specifying SIZE AUTO allows the database to automatically decide which columns need histograms.
You can verify histogram statistics in Oracle by querying the DBA_HISTOGRAMS table. This verification step ensures that the histograms are correctly created and provide accurate data distribution information.
Best Practices for Stats Gathering in Oracle
To maintain performance, it is advisable to gather statistics on frequently modified tables weekly or monthly. The DBMS_AUTO_TASK_IMMEDIATE package allows manual execution of the same statistics gathering job as the automated nightly job. Regularly gathering optimizer statistics using the DBMS_STATS package enhances query performance.
Using reporting mode can help database administrators make informed decisions about when to gather statistics. Configuring automatic statistics collection in Oracle helps prioritize database objects requiring statistics. Oracle recommends gathering statistics regularly to reflect changes in data volume and structure. Scheduling statistics gathering is best managed through scripts or job schedulers.
If automatic statistics gathering is disabled, having a good manual statistics collection strategy is essential. In Oracle, the infrastructure for automated maintenance tasks schedules these tasks to run independently, typically during designated maintenance windows.
Following these best practices ensures that your Oracle database remains efficient and performant, with up-to-date and accurate statistics guiding the optimizer’s decisions.
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Frequently Asked Questions
Why is gathering statistics important for Oracle databases?
Gathering statistics is essential for optimizing query performance. It allows the Oracle optimizer to create efficient execution plans based on accurate data, ultimately enhancing the database’s overall performance.
How often should I gather statistics for frequently modified tables?
Gathering statistics weekly or monthly is advisable to maintain optimal performance for frequently modified tables. This practice ensures the database can make informed decisions about query execution plans.
How can I check for stale statistics in Oracle?
To check for stale statistics in Oracle, you should query the DBA_TAB_STATISTICS view and monitor the STALE_STATS column. This will help you identify tables that need their statistics updated.
What are the benefits of using incremental statistics maintenance?
Incremental statistics maintenance enhances performance by efficiently scanning only the changed partitions, leading to quicker and more effective statistics gathering. This approach ultimately optimizes resource utilization and reduces processing time.
