What is indexing in SQL?

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Indexing in SQL is a technique used to speed up data retrieval from a database table by creating a data structure (similar to an index in a book) that allows the database engine to find rows more efficiently. Instead of scanning every row (full table scan), the database uses the index to quickly locate the required records.

🔹 How It Works

• An index is usually created on one or more columns.

• Internally, databases often use B-Trees or Hash structures to store index entries.

• When a query with WHERE, JOIN, or ORDER BY is executed, the index helps in locating rows faster.

🔹 Types of Indexes

1. Primary Index – Automatically created when a primary key is defined.

2. Unique Index – Ensures all values in a column are unique.

3. Clustered Index – Determines the physical order of rows in a table (one per table).

4. Non-clustered Index – Stores a separate structure pointing to actual data rows (multiple allowed).

5. Composite Index – Created on multiple columns.

🔹 Example

CREATE INDEX idx_name ON Employees(name);

Now queries filtering by name will be faster.

🔹 Trade-offs

• Indexes improve read performance, but slow down INSERT, UPDATE, DELETE because indexes must also be updated.

• They also consume extra storage space.

✅ Summary: Indexing is essential for optimizing query performance, but should be used wisely to balance speed and storage overhead.

Read More :

What is a primary key and foreign key?

What is normalization in databases?

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