Master MySQL Indexes: Key Concepts, Pros, Cons, and When to Use Them

What is an index?

An index is a data structure used by the storage engine to speed up access to database tables.

Advantages and disadvantages of indexes

Advantages:

Accelerates data lookup speed

Adding an index to fields used for sorting or grouping can speed up grouping and sorting

Speeds up joins between tables

Disadvantages:

Creating an index consumes physical storage space

It reduces the efficiency of insert, delete, and update operations because each modification must dynamically maintain the index , lengthening operation time

Purpose of indexes

Data is stored on disk; without an index, querying loads all data into memory and scans sequentially, causing many disk reads. With an index, only 2‑4 disk reads are needed (B+ tree height), dramatically improving query speed.

When to create an index

Fields frequently used in queries

Fields often used in joins, to speed up join operations

Fields frequently sorted, as indexes are pre‑sorted and accelerate sorting queries

When not to create an index

Fields not used in where conditions are unsuitable for indexing

Tables with very few records

When frequent insert, delete, or update operations occur

Columns involved in calculations are not suitable for indexing

Columns with low selectivity (e.g., gender) are not suitable for indexing