Composite Index FAQS

1. What is a Composite Index in Oracle?

• A Composite Index in Oracle is an index created on two or more columns of a table. It helps improve the performance of queries that filter or sort data using multiple columns. A Composite Index allows Oracle to efficiently search, retrieve, and sort data by combining values from multiple columns.

2. How is a Composite Index different from a Simple Index?

• A Simple Index is created on a single column, whereas a Composite Index is created on two or more columns. A Composite Index optimizes queries that use multiple columns in their WHERE clause, JOIN conditions, or ORDER BY statements.

3. When should I use a Composite Index?

• You should use a Composite Index when your queries frequently filter or sort data based on more than one column. It is particularly useful for multi-column queries, joins, and range queries that involve multiple columns.

4. How does Oracle use a Composite Index in queries?

• Oracle uses a Composite Index to efficiently retrieve rows for queries that use one or more of the indexed columns. The order of the columns in the index matters. Queries that filter on the first column of the index will benefit the most. If the query filters on a column later in the index, it may not benefit as much.

5. Can a Composite Index be created on any set of columns?

• Yes, a Composite Index can be created on any set of columns, but it is most beneficial when the columns are frequently used together in queries. The columns with the highest cardinality (distinct values) should typically be placed first in the index for optimal performance.

6. How do I create a Composite Index in Oracle?

• You can create a Composite Index using the CREATE INDEX statement. For example:

CREATE INDEX idx_emp_dept_name ON employees (department_id, last_name);

This creates an index on the department_id and last_name columns.

7. What is the order of columns in a Composite Index?

• The order of columns in a Composite Index is crucial. Oracle uses the columns in the exact order in which they are defined in the index. Queries that filter or sort by the columns in the same order will benefit the most from the index. For example, an index on (A, B) can be used for queries that filter by both A and B or just A, but not for queries that filter only by B.

8. Can a Composite Index improve performance for range queries?

• Yes, a Composite Index can improve performance for range queries, especially if the query involves the first column of the index. For example:

SELECT * FROM employees

WHERE department_id = 10 AND hire_date BETWEEN '2020-01-01' AND '2020-12-31';

A composite index on (department_id, hire_date) would optimize this query.

9. What are the advantages of using a Composite Index?

• Improved Query Performance: Speeds up queries that filter, sort, or join on multiple columns.
• Reduces Full Table Scans: Avoids full table scans and speeds up data retrieval.
• Enforces Uniqueness: A Composite Unique Index ensures that the combination of indexed columns is unique across rows.

10. What are the disadvantages of using a Composite Index?

• Storage Overhead: A composite index consumes additional storage space, especially with multiple indexed columns.
• Maintenance Overhead: Inserts, updates, and deletes require additional processing to maintain the index, which can affect performance.
• Not Effective for Low-Cardinality Columns: Composite indexes may not be effective for columns with few distinct values (low cardinality).

11. How does Oracle handle updates when a Composite Index is involved?

• When a row is inserted, updated, or deleted, Oracle updates the corresponding entries in the Composite Index to maintain data consistency. If any of the indexed columns are changed, the index entries are modified accordingly.

12. Can I drop a Composite Index in Oracle?

• Yes, you can drop a Composite Index using the DROP INDEX statement:

DROP INDEX idx_emp_dept_name;

13. How does the Composite Index help in join operations?

• A Composite Index can speed up JOIN operations, particularly when the join condition involves multiple columns. If a query joins two tables on columns that are indexed in the same order, the index can be used to find matching rows quickly.

14. Can I create a Composite Index on NULL values?

• Yes, Composite Indexes can be created on columns that contain NULL values. However, NULL values are treated as distinct, and multiple NULL values can be stored in the index.

15. Can I use a Composite Index for GROUP BY or ORDER BY operations?

• Yes, a Composite Index can improve performance for GROUP BY or ORDER BY operations, especially if the columns used in the query match the columns in the index. Oracle can use the index to efficiently sort or group the data.

16. Can a Composite Index be used if the query references only some of the indexed columns?

• Yes, Oracle can still use a Composite Index if the query references only the first column(s) in the index. However, queries that reference columns later in the index may not benefit as much.

17. What is the impact of using a Composite Index on performance?

• Positive Impact: Composite indexes improve query performance for multi-column searches, joins, and sorting.
• Negative Impact: They add storage overhead and may slow down INSERT, UPDATE, and DELETE operations due to the need to maintain the index.

18. Can a Composite Index improve performance for queries with BETWEEN or LIKE conditions?

• Yes, a Composite Index can improve performance for range-based queries (BETWEEN, >=, <=) or pattern-matching queries (LIKE) if the indexed columns are involved in the query's condition.

19. Can a Composite Index be created on a view?

• No, indexes cannot be created directly on views in Oracle. However, if the view is based on a table with an index, Oracle may still use the underlying index when querying the view.

20. How can I check if a Composite Index is being used by a query?

• You can use the EXPLAIN PLAN statement to check if Oracle is using a Composite Index for a particular query:

EXPLAIN PLAN FOR

SELECT * FROM employees WHERE department_id = 10 AND last_name = 'Smith';