Indexing

Introduction

An Index in a database is a data structure (typically a B-Tree or Hash Table) that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space.

Think of an index like the index at the back of a book: instead of reading every single page to find a specific word (a full table scan), you check the index to find the exact page numbers instantly.

Creating an Index

The CREATE INDEX statement is used to create indexes in tables.

CREATE INDEX idx_lastname
ON Persons (LastName);

You can also create an index on a combination of columns (a composite index):

CREATE INDEX idx_pname
ON Persons (LastName, FirstName);

Unique Indexes

A unique index ensures that two rows cannot have the same index value. Primary keys automatically get a unique index.

CREATE UNIQUE INDEX idx_email
ON Users (Email);

When to use Indexes

Indexes drastically speed up SELECT queries, especially those with WHERE, JOIN, or ORDER BY clauses.

However, you should not index every column. Why?

1. Slower Writes: Every time you INSERT, UPDATE, or DELETE a row, the database must also update the index.
2. Storage Space: Indexes take up physical disk space.
3. Small Tables: If a table has only a few hundred rows, an index might actually be slower than a full table scan.

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