Index in Oracle Database

An index in Oracle Database is a database structure that provides a fast access path to specific rows in a table, similar to how a book’s index helps you quickly find specific topics. Instead of scanning an entire table (like reading a book cover-to-cover), Oracle can use indexes to locate the desired data much more efficiently.

Key aspects of Oracle indexing:

1. Structure and Function

• Indexes are separate structures from tables
• They contain the indexed column values and corresponding ROWIDs
• Oracle automatically maintains indexes when data in the referenced table changes
• They consume additional storage space but significantly improve query performance

2. Common Index Types:

• B-tree Index (default): Best for columns with high selectivity
• Bitmap Index: Efficient for columns with low selectivity
• Function-Based Index: Based on expressions or functions of columns
• Reverse Index: Useful for sorting in reverse order
• Unique Index: Enforces uniqueness of column values

3. When to Use Indexes:

• Primary key columns (Oracle creates these automatically)
• Foreign key columns
• Columns frequently used in WHERE clauses
• Columns used in JOIN conditions
• Columns frequently used in ORDER BY or GROUP BY

4. When to Avoid Indexes:

• Small tables where full table scans are faster
• Columns with low selectivity
• Columns that are frequently updated
• Tables with heavy INSERT activity

5. Creating Indexes:

Example of creating a basic index:

CREATE INDEX employee_lastname_idx 
ON employees (last_name);

Example of creating a composite index:

CREATE INDEX emp_name_salary_idx 
ON employees (last_name, salary);

Example of creating a unique index:

CREATE UNIQUE INDEX emp_email_idx 
ON employees (email);

6. Index Maintenance Considerations:

• Regularly analyze indexes to update statistics
• Monitor index usage and remove unused indexes
• Rebuild fragmented indexes when necessary
• Consider partitioning indexes for very large tables

7. Performance Impact:

• Speeds up SELECT queries
• Slows down INSERT, UPDATE, and DELETE operations
• Requires additional disk space
• Can improve sort operations
• Helps enforce data integrity (unique indexes)

Understanding and properly implementing indexing is crucial for Oracle Database performance optimization. The key is finding the right balance between query performance improvement and maintenance overhead.