Introduction

         Indexes are a fundamental aspect of Oracle SQL that can significantly enhance the performance of database queries. It provide direct ,fast access to rows. In this blog post, we will deep dive into indexes in Oracle SQL , exploring what they are, how they work, and their different types. By the end of this guide, you’ll have a solid understanding of indexes and how to leverage them to optimize your SQL queries.

Understanding Indexes

       Indexes are database objects designed to improve query performance by providing faster data retrieval. They act like a table of contents in a book, allowing the database engine to locate data more efficiently. Instead of scanning the entire table, Oracle SQL can use an index to pinpoint the exact rows that satisfy a query’s criteria.

Types of Indexes

Oracle SQL offers several types of indexes, each with specific use cases:

1.  B-Tree Indexes:

• B-Tree (Balanced Tree) indexes are the most common type (default index).
• It is suitable for the column containing high numbers of unique values(High Cardinality).
• It is an ordered list of values divided into range .
• Example:

1. 1. account number in banking system.
   2. Student registered number.
   3. Employee id.

• Creating and Managing Indexes
  • Create Index:
    • Use this SQL statement to create an index. You need to specify the table and column(s) on which the index is created and choose the index type. Use below SQL command to create an index .

    • Syntax:
      CREATE INDEX index_name on table_name(column_name);
      
      Example:
      CREATE INDEX idx_btree_emp on emp(emp_no);

• • Drop Index:
    • To remove an index, you can use the DROP INDEX statement. This operation should be performed with caution, as it can impact query performance. Use below SQL command to drop an existing index.

    • Syntax:
      DROP INDEX index_name;
      
      Example:
      DROP INDEX idx_btree_emp;

• • Alter Index:
    • The ALTER INDEX statement allows you to modify an existing index, such as changing its storage options or rebuilding it for performance reasons.

    • Syntax:
      ALTER INDEX current_index_name
      RENAME TO new_indexname;
      
      Example:
      ALTER INDEX btree_ind_emp
      RENAME TO btree_ind_emp_new;

2. Bitmap Indexes:

• Bitmap indexes are ideal for columns with a low cardinality (few distinct values).
• Very less DML(Data Manipulation Language) activity or preferably read only tables.
• It often used in data warehousing scenarios. They store a bitmap for each possible value, indicating which rows contain that value.
• Example:
  • Movie Rating(1-10)
  • Color Category(Red, Blue, Green, Yellow etc.)
• Creating and Managing Indexes
  • Create Index:
    • Use this SQL statement to create an index. You need to specify the table and column(s) on which the index is created and choose the index type. Use below SQL command to create an index .

    • Syntax:
      CREATE BITMAP INDEX index_name ON table_name(column_name);
      
      Example:
      CREATE BITMAP INDEX idx_bitmap_dept_no ON t_dept_det(dept_no);

• • Drop Index:
    • To remove an index, you can use the DROP INDEX statement. This operation should be performed with caution, as it can impact query performance. Use below SQL command to drop an existing index.

    • Syntax:
      DROP INDEX index_name;
      
      Example:
      DROP INDEX idx_btree_emp;

• • Alter Index:
    • The ALTER INDEX statement allows you to modify an existing index, such as changing its storage options or rebuilding it for performance reasons.

    • Syntax:
      ALTER INDEX current_index_name
      RENAME TO new_indexname;
      
      Example:
      ALTER INDEX idx_bitmap_dept_no
      RENAME TO idx_bitmap_dept_number ;

3. Function-Based Indexes:

• Function-based indexes allow you to index the result of a function applied to a column.
• This is useful for optimizing queries involving expressions or calculations.
• When the database processes the insert and update statement, however , it must still evaluate the function to process the statement.
• Creating and Managing Indexes
  • Create Index:
    

    • Syntax:
      CREATE INDEX idx_name ON table_name(function(column_name));
      
      Example:
      CREATE INDEX idx_fun_emp_name ON emp(UPPER(emp_name));

  • Drop Index:

    • Syntax:
      DROP INDEX index_name;
      
      Example:
      DROP INDEX idx_fun_emp_name ;

  • Alter Index:

    • Syntax:
      ALTER INDEX current_index_name
      RENAME TO new_indexname;
      
      Example:
      ALTER INDEX idx_fun_emp_name
      RENAME TO idx_fun_upper_emp_name ;

4. Reverse Key Indexes:

• Reverse key index is type of B-Tree index that physically reverse the byte of each index key while keeping the column order.
• It is used to avoid index block contention when many rows are inserted.
• Value swap and then stored, No sorting done for this type of index.
• Creating and Managing Indexes
  • Create Index:

    • Syntax:
      CREATE INDEX index_name on table_name(column_name) REVERSE;
      
      Example:
      CREATE INDEX idx_btree_emp on emp(sal) REVERSE;

  • Drop Index:

    • Syntax:
      DROP INDEX index_name;
      
      Example:
      DROP INDEX idx_btree_emp;

  • Alter Index:

    • Syntax:
      ALTER INDEX current_index_name
      RENAME TO new_indexname;
      
      Example:
      ALTER INDEX idx_btree_emp
      RENAME TO idx_btree_emp_new;

5. Composite Indexes:

• An index created on multiple table columns is known as a multi-column or composite index.
• The main purpose of composite indexes is to improve the performance of data retrieval for SELECT statements when filtering on all, or some, of the composite index columns.
• Creating and Managing Indexes
  • Create Index:

    • Syntax:
      CREATE INDEX index_name on table_name(multiple_column_name);
      
      Example:
      CREATE INDEX idx_comp_emp ON t_emp_det(sal,dept_id);

  • Drop Index:

    • Syntax:
      DROP INDEX index_name;
      
      Example:
      DROP INDEX idx_comp_emp;

  • Alter Index:

    • Syntax:
      ALTER INDEX current_index_name
      RENAME TO new_indexname;
      
      Example:
      ALTER INDEX idx_comp_emp
      RENAME TO idx_comp_emp_sal_deptno;

Best practices for using indexes in Oracle SQL

To maximize the benefits of indexes in Oracle SQL, consider the following best practices:

• Index Selectively: 
  1. Index only the columns that are frequently used in WHERE clauses and JOIN conditions.
  2. Over-indexing can lead to performance degradation.

• Avoid Indexing Small Tables:
  1. Indexes may not provide significant benefits for very small tables, as full table scans can be faster in such cases.

• Be Mindful of Table Updates:
  1. Keep in mind that indexes need to be updated when the underlying table data changes, so excessive inserts, updates, or deletes can impact performance.

• Use the Query Optimizer:
  1. Trust Oracle’s query optimizer to choose the best execution plan.
  2. It often performs better than manually forcing index use.

Conclusion

Indexes are a vital tool for optimizing query performance in Oracle SQL. Understanding the types of indexes, how to create and manage them, and best practices for their usage is essential for maintaining a high-performing database. When used judiciously, indexes can significantly enhance the speed and efficiency of your SQL queries, making them an indispensable feature in the world of Oracle database management.

Thank You!!  Happy Learning!!