What do you mean the Oracle Optimizer won't use my Index? John Mullins

Transcription

1 What do you mean the Oracle Optimizer won't use my Index? John Mullins

2 Presenter John Mullins Themis Inc. 32+ years of Oracle experience Oracle Certified Professional DBA Certified Technical Trainer Over 320 classes taught

3 Themis Inc. More than 25 years in the industry More than 90,000 students have attended a Themis training event Courses: DB2, SQL Server, Oracle, Unix, Linux, Java, Web Development,.NET and many more

4 Related Themis Courses Oracle SQL Optimization for Developers and DBA's

5 Webinar Objectives To become familiar with the Oracle Optimizer and its decision making process when deciding on index usage

6 Factors Some factors that influence the Optimizer when considering indexes: Code Statistics Clustering Factor Parameter Settings Estimated Cardinality

7 Index - Yes SELECT last_name FROM employee WHERE department_id = 10;

8 Index - No SELECT last_name FROM employee WHERE department_id <> 10;

9 Index - Yes SELECT last_name, department_id FROM employee WHERE last_name like 'S%';

10 Index - No SELECT last_name, department_id FROM employee WHERE UPPER(last_name) like 'S%';

11 Index - No SELECT last_name, department_id FROM employee WHERE last_name like '%s%';

12 Index - Yes create index ix7 on employee (department_id, job_cd); SELECT last_name FROM employee WHERE department_id = 10;

13 Index - Yes create index ix7 on employee (department_id, job_cd); Index Skip Scan SELECT last_name FROM employee WHERE job_cd = 20;

14 Index - No alter session set optimizer_index_cost_adj = 1000; SELECT last_name FROM employee WHERE department_id = 10;

15 Statistics The Oracle Optimizer examines all of the possible plans for a SQL statement and picks the one with the lowest cost The cost represents the estimated resource usage for a given plan

16 Statistics In order for the optimizer to accurately determine the cost for an execution plan it must have information about all of the objects accessed in the SQL statement Inaccurate or missing statistics can cause the optimizer to choose a bad plan Check the all_tables, all_indexes and all_tab_partitions views to see the statistics for a specific object

17 Clustering Factor A number that represents the correlation between the order of the index and the order of the corresponding table Tells the cost-based optimizer how clustered or scattered the data in the table is with respect to the index

18 Clustering Factor If the count is close to the number of blocks in the table then the index is well ordered If the count is close to the number of rows in the table then the index is less well ordered The clustering factor for an index can be viewed from the all_indexes view

19 Parameter Settings Some parameters that can influence the optimizer when deciding on indexes db_file_multiblock_read_count optimizer_index_cost_adj optimizer_index_caching optimizer_mode

20 Estimated Cardinality Cardinality is the estimated number of rows a step within a query s execution plan will return It is important for the cardinality estimates to be as accurate as possible as they influence all aspects of the execution plan from the access method, to the join order

21 Summary There are many factors that can influence the Oracle Optimizer when its deciding on index usage Questions?

22 Upcoming Webinars DB2 SQL PL Array Parameters and Variables Wednesday October 11 11:30am ET CICS/DB2 Interface Thursday November 16 11:30am ET

23 For More Information Visit the Themis web site John Caccavale To get a copy of the presentation: Thank you for attending. Have a good day.