Understanding Indexing in Databases: Boost Your Query Performance

When working with large datasets, performance becomes a critical factor. One of the most effective ways to optimize database performance is through indexing. Let’s explore what indexing is, how it works, and why it can dramatically speed up your queries.

What Is Indexing?

Indexing is a data structure technique used to improve the speed of data retrieval operations on a database table. It reduces the number of disk reads required during query execution by allowing the database to locate data without scanning every row.

Think of an index like the table of contents in a book — instead of flipping through every page to find a topic, you go straight to the right section.

What Does Indexing Do?

Without an index, a database performs a full table scan, reading each row one by one to find matches — this is slow and inefficient, especially as the table grows.

The Problem: Unordered Table

Imagine you have a table called index_test that looks like this (unordered):
📷 View Example Table (Before Indexing)

Now, you run the following query:

SELECT
  company_id,
  units,
  unit_cost
FROM
  index_test
WHERE
  company_id = 18;

In this case, the database must search all 17 rows, top to bottom, comparing each company_id to 18. This process is linear and inefficient.

The Solution: Add an Index

By adding an index on the company_id column:

CREATE INDEX idx_company_id ON index_test(company_id);

The table is internally reorganized in a way that allows fast, binary-like search operations.

View Table with Indexing Applied

Now, when the database processes the query:

• It locates the first row with company_id = 18.
• Returns the relevant data.
• Continues until it encounters a row where company_id is greater than 18.
• At that point, it stops searching — because no more matches will be found due to the ordered index.

Key Takeaways

• Indexing improves query performance by minimizing the need for full table scans.
• Indexes work best on columns frequently used in WHERE, JOIN, ORDER BY, or GROUP BY clauses.
• Over-indexing can hurt performance for INSERT, UPDATE, and DELETE operations.

Bonus Tip

You can view existing indexes on a table using this command (in MySQL):

SHOW INDEX FROM index_test;

Conclusion

Indexing is an essential optimization strategy in relational databases. By strategically indexing your data, especially on columns that are frequently queried, you can dramatically enhance your application’s performance and responsiveness.