SQL - Indexing Strategies and Performance Tuning in SQL

1. What is Indexing?

An index in SQL is a data structure that improves the speed of data retrieval from a database table.
It works similarly to an index in a book — instead of scanning every page, you go directly to the section you need.

Without indexes, the database may need to check every row (called a full table scan). With indexes, it can locate data much faster.

2. Why are Indexes Important?

  • Speed up search operations

  • Improve query performance

  • Reduce database workload

  • Essential for large datasets

  • Support faster sorting and filtering

However, indexes also use storage space and can slow down insert or update operations because they must be maintained.

3. Creating an Index

Basic syntax:

CREATE INDEX index_name
ON table_name(column_name);

Example:

CREATE INDEX idx_customer
ON customers(customer_id);

This helps queries that search by customer_id run faster.

4. Types of Indexes

Single-column index
Created on one column.

Composite index
Created on multiple columns.

CREATE INDEX idx_name_dept
ON employees(name, department);

Unique index
Ensures values are unique.

Clustered index
Determines physical storage order of rows (varies by database system).

Non-clustered index
Separate structure that points to table data.

5. Indexing Strategies

Index frequently searched columns
Columns used in WHERE conditions benefit from indexing.

Index join columns
Columns used in joins can improve performance.

Avoid indexing very small tables
Full scan may already be fast.

Avoid over-indexing
Too many indexes slow data updates.

Choose appropriate column order
In composite indexes, order affects efficiency.

6. Performance Tuning Concepts

Monitor slow queries
Identify which queries need improvement.

Review execution plans
Understand how database accesses data.

Remove unused indexes
Free resources and improve updates.

Optimize schema design
Proper table structure improves speed.

Use appropriate data types
Smaller types reduce storage and processing.

7. Example Scenario

Query without index:

SELECT * FROM orders
WHERE order_id = 1001;

If order_id has no index, the database scans entire table.

After indexing:

CREATE INDEX idx_order
ON orders(order_id);

The database locates the row quickly using the index.

Summary

Indexing strategies focus on placing indexes where they provide the most benefit while avoiding unnecessary overhead. Performance tuning involves monitoring queries, analyzing execution behavior, and adjusting database structures for efficiency. Together, they help databases handle large workloads smoothly and quickly.