Optimizing Performance with Oracle Database: Gathering Schema and Table Statistics
Optimizing Performance with Oracle Database: Gathering Schema and Table Statistics
In database management, ensuring optimal performance is crucial for effective data handling and query execution. One of the foundational tasks for maintaining an Oracle Database system is gathering statistics, which helps the Oracle optimizer make informed decisions about the most efficient way to execute SQL statements. This article will explore how to gather stats for a single schema in Oracle Database using DBMS_STATS, breaking down the relevant code snippets, their purposes, and their implications for performance tuning.
Sample SQL Command
1Gather stats for a single schema...
2
3 execute dbms_stats.gather_schema_stats('SCOTT');
4
5Or...
6
7 execute dbms_stats.gather_schema_stats( -
8 ownname => 'SCOTT', -
9 estimate_percent => 1, -
10 method_opt => 'FOR ALL COLUMNS SIZE 1',-
11 cascade => TRUE);
12
13You can let oracle come up with the estimate figure by using dbms_stats.auto_sample_size
14
15or...
16 execute dbms_stats.gather_schema_stats( -
17 ownname => 'SYS', -
18 cascade => TRUE);
19
20Table statistics
21 exec dbms_stats.gather_table_stats('<owner>', '<table_name>');
Purpose of Gathering Statistics
Statistics collection involves gathering metadata about database objects, such as tables and indexes. The Oracle optimizer uses this data to determine the best execution plan for SQL queries. The optimizer may select suboptimal query plans without accurate statistics, leading to poor performance.
Code Breakdown
Gathering Stats for a Single Schema
To gather statistics for an entire schema (e.g., SCOTT), you can execute the following command:
1execute dbms_stats.gather_schema_stats('SCOTT');
Breakdown:
dbms_stats.gather_schema_stats: This procedure collects statistics for all tables and indexes in the specified schema.'SCOTT': This argument specifies the schema whose statistics need to be gathered.
Customizing Schema Statistics Gathering
For more control over the statistics gathering process, you can customize the command with additional parameters:
1execute dbms_stats.gather_schema_stats( -
2 ownname => 'SCOTT', -
3 estimate_percent => 1, -
4 method_opt => 'FOR ALL COLUMNS SIZE 1',-
5 cascade => TRUE);
Breakdown:
ownname => 'SCOTT': This specifies the owner of the schema.estimate_percent => 1: This parameter indicates that only 1% of the data should be sampled. Adjusting this affects the accuracy and performance; a lower percentage may speed up the process but could yield less accurate statistics.method_opt => 'FOR ALL COLUMNS SIZE 1': This option specifies how the histogram should be collected. Here, it indicates that all columns will be included with a histogram size of 1.cascade => TRUE: This causes statistics for indexes associated with the tables to also be collected.
Using Auto Sample Size
For convenience, Oracle can automatically determine the estimate size using:
1execute dbms_stats.gather_schema_stats( -
2 ownname => 'SCOTT', -
3 cascade => TRUE, -
4 auto_sample_size => TRUE);
Using auto_sample_size => TRUE allows Oracle to determine the most effective sample size for gathering statistics, balancing performance and accuracy without manual estimation.
Gathering Statistics for SYS Schema
Sometimes, it may be necessary to gather statistics specifically for the SYS schema (the user that owns the Oracle data dictionary) as follows:
1execute dbms_stats.gather_schema_stats( -
2 ownname => 'SYS', -
3 cascade => TRUE);
Gathering Statistics for Individual Tables
To gather statistics for a specific table within a schema, you can use:
1exec dbms_stats.gather_table_stats('<owner>', '<table_name>');
Breakdown:
<owner>: Replace this with the schema name where the table resides.<table_name>: Replace this with the name of the table for which you are collecting stats.
Key Points
- Critical for Optimization: Gathering statistics is essential for the Oracle optimizer to perform well.
- Performance Impact: The method of gathering (e.g., sample size) can significantly impact the performance of your operations and accuracy of statistics.
- Regular Maintenance: Schedule regular statistics gathering as part of database maintenance to ensure optimal execution plans are in use.
Insights
- Time Zone Consideration: Gather statistics during off-peak hours to minimize the impact on performance.
- Monitoring Statistics: After gathering statistics, monitor query performance to assess whether the new statistics have improved execution plans.
- Automation: Consider using Oracle’s DBMS_JOB or scheduler for automated statistics collection.
Conclusion
Gathering schema and table statistics in Oracle Database is fundamental for ensuring optimal performance. Familiarity with the various options available through the DBMS_STATS package empowers database administrators to maintain their systems effectively and enhance query execution efficiency.