What Are SQL Indexes Used For?

SQL indexes are fundamental database structures that enhance the performance and efficiency of database operations.

They serve as a roadmap to quickly locate and retrieve data from tables, significantly improving query execution speed.

In this article, we explore the purpose and benefits of SQL indexes, their types, and best practices for their usage, along with examples.

Table of Contents

  1. Introduction
  2. Understanding SQL Indexes
  3. Types of SQL Indexes
  4. Benefits of SQL Indexes
  5. Creating and Managing SQL Indexes
  6. Best Practices for Using SQL Indexes
  7. Conclusion


SQL indexes are database structures designed to speed up the retrieval of data from database tables.

They are crucial for optimizing query performance, particularly when dealing with large datasets.

Without indexes, SQL databases would have to scan entire tables, resulting in slow and inefficient queries.

Understanding SQL Indexes

In the context of SQL databases, an index is similar to an index in a book.

It is a data structure that provides a quick way to look up records based on the values of one or more columns.

Indexes are created from one or more columns of a table and store a copy of the data in a format that allows for efficient searching and retrieval.

Types of SQL Indexes

Single-Column Index

A single-column index is created on a single column of a table. It speeds up queries that filter or sort data based on that specific column.

Single-column indexes are the simplest type of indexes and are commonly used in database systems.


Let's say you have a products table, and you want to create a single-column index on the product_name column to improve search performance:

CREATE INDEX product_name_index ON products(product_name);

Multi-Column Index (Composite Index)

A multi-column index, also known as a composite index, is created on two or more columns of a table.

It is useful when queries involve conditions or sorting on multiple columns. Multi-column indexes can improve performance for complex queries.


In the orders table, you can create a composite index on the customer_id and order_date columns to speed up queries that filter and sort based on both columns:

CREATE INDEX customer_order_index ON orders(customer_id, order_date);

Unique Index

A unique index enforces the uniqueness of values in the indexed column(s).

It ensures that no two rows can have the same values in the indexed column(s). Unique indexes are commonly used for primary keys, ensuring data integrity.


To enforce uniqueness on the username column in the users table, you can create a unique index:

CREATE UNIQUE INDEX username_unique_index ON users(username);

Full-Text Index

A full-text index is used for efficient searching within text or character-based columns, such as those containing large text documents or articles.

Full-text indexes enable advanced text-based searches, including keyword searches and phrase matching.


In a posts table containing blog posts, you can create a full-text index on the post_content column for improved text-based searching:

CREATE FULLTEXT INDEX post_content_fulltext_index ON posts(post_content);

Benefits of SQL Indexes

SQL indexes offer numerous benefits, including:

Creating and Managing SQL Indexes

Creating an Index

To create an index, you use the CREATE INDEX statement. The syntax may vary slightly depending on the database management system you are using, such as MySQL, PostgreSQL, or SQL Server.


To create a new index on the email column of the contacts table in PostgreSQL:

CREATE INDEX email_index ON contacts(email);

Modifying an Index

You can modify an index to change its structure or properties using the ALTER INDEX statement or equivalent.

Common modifications include adding or removing columns from a composite index.


In SQL Server, you can add a new column to an existing index as follows:

ALTER INDEX [index_name] ON [table_name]
ADD ([new_column]);

Dropping an Index

To remove an index from a table, you use the DROP INDEX statement.

This action permanently deletes the index, and you should exercise caution, especially in a production environment.


To drop an index named product_name_index from the products table in MySQL:

DROP INDEX product_name_index ON products;

Best Practices for Using SQL Indexes

Choose Indexes Wisely

Select the columns for indexing carefully. Consider the specific queries that will benefit from the index and avoid over-indexing, as excessive indexes can impact insert and update performance.

Regularly Maintain Indexes

Indexes may need periodic maintenance, such as rebuilding or defragmentation, to ensure they remain efficient over time.

Monitor Index Performance

Regularly monitor the performance of your indexes to identify potential issues and make necessary adjustments.

Be Cautious with Over-Indexing

While indexes can significantly improve query performance, creating too many indexes can have a negative impact on insert, update, and delete operations. Be mindful of the trade-off between read and write performance.


SQL indexes are essential tools for optimizing database performance by facilitating efficient data retrieval. Understanding the types of indexes, their benefits,

and best practices for their usage is crucial for maintaining a well-performing database system. By implementing indexes judiciously and regularly monitoring their performance, you can ensure that your database queries run swiftly and effectively.