Composite Indexes in Oracle Database
Introduction
A composite index, also known as a multi-column index, is an index created on multiple columns of a table. Unlike single-column indexes, composite indexes can improve the performance of queries that filter or sort based on multiple columns simultaneously.
Creating Composite Indexes
You can create a composite index using the CREATE INDEX statement and specifying multiple columns in the ON clause.
Syntax:
CREATE INDEX index_name ON table_name (column1, column2, ...);
Example:
CREATE INDEX idx_emp_dept_salary ON employees (department_id, salary);
In this example, idx_emp_dept_salary is a composite index created on the department_id and salary columns of the employees table.
Advantages of Composite Indexes
Optimization for Multi-Column Queries
Composite indexes are particularly useful for optimizing queries that filter or sort on multiple columns. They allow the database engine to use a single index structure to satisfy complex search criteria.
Example Query:
SELECT * FROM employees WHERE department_id = 10 AND salary > 50000;
In this query, the idx_emp_dept_salary index can be used to efficiently find employees in department 10 with a salary greater than 50000.
Improved Performance for Sorting and Grouping
Composite indexes can also improve performance for sorting (ORDER BY) and grouping (GROUP BY) operations when the indexed columns are used in these operations.
Example Sort Query:
SELECT * FROM employees ORDER BY department_id, salary;
The composite index idx_emp_dept_salary can help sort the results of this query more efficiently.
Using Composite Indexes
Column Order in the Index
The order of columns in a composite index is important. Oracle uses the index in the order of columns specified to optimize queries. The column order can affect how queries are optimized.
Example:
- Index on (department_id, salary) : Optimizes queries filtering by department_id and more precisely by salary within each department_id.
- Index on (salary, department_id) : Optimizes queries filtering primarily by salary, with a secondary filter by department_id.
Example Query Optimized by (salary, department_id) :
SELECT * FROM employees WHERE salary > 50000 AND department_id = 10;
If the index is on (salary, department_id), it may be more effective for this query.
Query Conjunction
Composite indexes are particularly effective when they cover all columns used in query conditions.
Example Covered Query:
SELECT * FROM employees WHERE department_id = 10 AND salary > 50000 AND hire_date > '2022-01-01';
If the composite index covers all these columns in the appropriate order, it can optimize this query.
Managing Composite Indexes
Dropping an Index
You can drop a composite index if it is no longer needed or if it negatively impacts performance.
Syntax:
DROP INDEX index_name;
Example:
DROP INDEX idx_emp_dept_salary;
Rebuilding an Index
Composite indexes can become fragmented over time. Rebuilding the index can help improve its performance.
Syntax:
ALTER INDEX index_name REBUILD;
Example:
ALTER INDEX idx_emp_dept_salary REBUILD;
Monitoring Index Usage
Monitor the usage of composite indexes to evaluate their effectiveness and impact on query performance.
Example:
SELECT * FROM V$OBJECT_USAGE WHERE INDEX_NAME = 'IDX_EMP_DEPT_SALARY';
Considerations
Index Costs
Composite indexes can consume more disk space and introduce overhead during data modification operations (inserts, updates, deletes). Balancing the benefits of indexing with the associated costs is crucial.
Index Design
When designing composite indexes, it is important to understand the data usage patterns and common queries. Indexing columns that are not frequently used in queries may not justify the cost.
Composite Indexes and Partially Covered Queries
Composite indexes are only effective when the columns used in queries are in the same order as defined in the index. If queries use only part of the index columns, the effectiveness may be reduced.
Conclusion
Composite indexes are a powerful tool for optimizing query performance in Oracle Database, particularly for queries involving multiple columns. By understanding how to create and manage these indexes effectively, you can significantly enhance query performance while balancing the associated costs.