Oracle

Getting Started with Indexes in Oracle Database

When dealing with large datasets in Oracle Database, query performance is key. One of the most powerful techniques to optimize SELECT queries is using indexes.

This blog post provides a comprehensive overview of Oracle indexes, including their types, syntax, examples, real-world use cases, and best practices to help developers and DBAs optimize data retrieval.

What is an Index?

An index in Oracle Database is an optional schema object that can be created on one or more columns of a table to improve query performance. Think of it like the index in a book: it helps you find topics quickly without reading every page.

Without an index, Oracle performs a full table scan, which can be time-consuming for large tables. Indexes allow Oracle to jump directly to the rows that match query conditions.

Why Use Indexes?

Indexes are crucial for,

  • Speeding up SELECT queries
  • Reducing I/O operations
  • Optimizing JOINs and WHERE clause filtering
  • Enforcing uniqueness with primary and unique key constraints

However, indexes come with a trade-off: they consume disk space and may slightly slow down INSERT, UPDATE, or DELETE operations due to maintenance overhead.

Types of Indexes in Oracle

Index Type Description
B-Tree Index Default index type; suitable for high-cardinality (many unique values) columns
Bitmap Index Ideal for low-cardinality (few unique values) columns; used in data warehouses
Unique Index Ensures uniqueness of values in one or more columns
Composite Index An index created on multiple columns
Function-Based Index Uses an expression or function in the index definition
Index-Organized Table (IOT) Stores table data sorted in the order of its primary key

Syntax and Examples

1. B-Tree Index (Default)

CREATE INDEX idx_emp_name
ON employees (first_name);

Used for quick lookup and range scans on high-cardinality columns.

2. Unique Index

CREATE UNIQUE INDEX idx_emp_email
ON employees (email);

Automatically created when you define a UNIQUE or PRIMARY KEY constraint.

3. Composite Index

CREATE INDEX idx_emp_dept_job 
ON employees (department_id, job_id);

Useful when queries filter on both columns together.

4. Bitmap Index

CREATE BITMAP INDEX idx_emp_gender 
ON employees (gender);

Best for data warehousing, not OLTP systems. Avoid frequent updates on such columns.

5. Function-Based Index

CREATE INDEX idx_emp_uppername 
ON employees (UPPER(first_name));

Enables Oracle to use the index when a function is applied in the WHERE clause.

6. Index-Organized Table (IOT)

CREATE TABLE students_iot (
    student_id NUMBER PRIMARY KEY,
    first_name VARCHAR2(50),
    last_name VARCHAR2(50),
    dob DATE
) ORGANIZATION INDEX;

Stores data sorted by the primary key instead of rowid. Excellent for lookups and range scans.

View Indexes on a Table

-- Select index details for the EMPLOYEES table
SELECT index_name, index_type, table_name
FROM user_indexes
WHERE table_name = 'EMPLOYEES';

-- Drop the index named idx_emp_name
DROP INDEX idx_emp_name;

Renaming an Index

ALTER INDEX idx_emp_name 
RENAME TO idx_employees_firstname;

Best Practices for Using Indexes

  • Use indexes on columns involved in frequent searches, joins, or filters
  • Avoid indexing columns with frequent updates
  • Don’t over-index: Indexes take up space and slow down data changes
  • Use EXPLAIN PLAN or AUTOTRACE to analyze query performance
  • Use composite indexes when queries use multiple columns in WHERE or JOIN conditions

Real-World Scenario

Imagine a table with 10 million employee records. Without an index, a query like,

SELECT * 
FROM employees 
WHERE email = '[email protected]';

Would perform a full table scan.

But with a unique index on the email column.

CREATE UNIQUE INDEX idx_emp_email ON employees (email);

Oracle can directly locate the record, improving performance drastically.

Conclusion

Indexes are essential for boosting the performance of your Oracle Database queries. By understanding the types of indexes and applying them appropriately, you can make your applications more responsive and efficient.

Tip. Always test and monitor your queries using tools like EXPLAIN PLAN to ensure the index is being used as expected.
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