Understanding SQL Types of Indexes: A Beginner’s Guide

Learn essential SQL index types: single-column, composite, unique, full-text, and bitmap indexes for efficient database queries

JackyNote ⭐️
4 min readOct 2, 2023

As a beginner in the world of SQL databases, one of the essential concepts to grasp is indexing. Indexes play a crucial role in improving the performance of database queries, and they are fundamental to efficient database design. In this article, we will explore the most common types of SQL indexes, providing examples, use cases, and insights into their algorithmic complexities to help beginners understand this vital topic.

What is an Index?

In SQL, an index is like a carefully organized reference book that helps you quickly find information within a database table without the need to read through the entire table. Without indexes, database queries would often require scanning the entire table, which can be slow and inefficient for large datasets.

Indexes work by creating a separate data structure that stores a subset of the table’s columns and a reference to the actual data. This organization allows SQL databases to find specific values much faster.

Common Types of SQL Indexes

Let’s delve into the most common types of SQL indexes that beginners should be aware of:

1. Single-Column Index

A single-column index is the simplest and most commonly used type of index. It focuses on a single column in a table, making it efficient for queries that filter or sort by that column.


CREATE INDEX idx_customer_id
ON customers (customer_id);
  • Use Case: If you frequently search for customers by their unique ID, a single-column index on the customer_id column can significantly improve query performance.
  • Algorithmic Complexity: Searching in a single-column index is typically O(log N), where N is the number of rows in the table.

2. Composite Index

A composite index, also known as a multi-column index, involves indexing multiple columns together. It is useful when queries involve filtering or sorting by a combination of these columns.


CREATE INDEX idx_last_name_first_name
ON employees (last_name, first_name)
  • Use Case: In an employee database, searching for employees by both last name and first name can benefit from a composite index on these two columns.
  • Algorithmic Complexity: Searching with a composite index is also O(log N) per indexed column, but the combined complexity depends on the selectivity of each column.

3. Unique Index

A unique index enforces the uniqueness of values in the indexed column(s), ensuring that no duplicate entries are allowed.


CREATE UNIQUE INDEX idx_unique_email
ON users (email);
  • Use Case: To ensure that user email addresses in a users table are unique, you can create a unique index on the email column.
  • Algorithmic Complexity: Searching in a unique index is typically O(log N), similar to a single-column index.

4. Full-Text Index

Full-text indexes are designed for efficient searching within large text columns, such as articles, blog posts, or comments. They enable keyword-based searches.


CREATE FULLTEXT INDEX idx_fulltext_content
ON articles (content);
  • Use Case: In a content management system, a full-text index on the content column allows for efficient keyword searching.
  • Algorithmic Complexity: The complexity of full-text search depends on the underlying text indexing algorithm but is often O(log N) or better.

5. Bitmap Index

A bitmap index is efficient for columns with low cardinality, meaning they have a small number of distinct values. It uses bitmaps to represent the presence or absence of rows that match a specific value.


CREATE BITMAP INDEX idx_bitmap_gender
ON employees (gender);
  • Use Case: When you have a column like gender with a limited number of values (e.g., “Male,” “Female,” “Non-Binary”), a bitmap index can be efficient for filtering by gender.
  • Algorithmic Complexity: Bitmap indexes offer constant-time (O(1)) lookups, making them very fast for certain types of queries.


In SQL databases, indexes are essential tools for improving query performance and optimizing database operations. Understanding the different types of indexes and their use cases is fundamental to designing efficient database systems. Whether you’re working with single-column indexes for basic queries or complex composite indexes for advanced searches, indexing is a crucial skill for any SQL developer. Moreover, knowing the algorithmic complexities can help you make informed decisions when designing your database schema and choosing the right indexes for your specific use cases.

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JackyNote ⭐️

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