SQL Indexes & Query Performance Explained

Indexes & Performance

1 min readPublished 22 Jul 2026

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As data grows, performance matters. An index is like a book's index — it lets the database find rows without scanning the whole table.

Interactive explainer

Database Indexes & Performance

Why some queries are instant and others crawl.

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1. No index = scan

Read every row.

Without an index, finding matching rows means a full table scan — the database reads every row. Fine for tiny tables, slow as data grows.

Check your understanding

1. What is the main cost of adding indexes?

2. What tool shows whether a query uses an index?

Keep learning

Indexes & Performance JOINs & Aggregation Microsoft DP-900

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The essentials

Without an index, a query does a full table scan; with one on the searched column, it jumps straight to the rows.
Trade-off: indexes speed up reads but slow down writes (INSERT/UPDATE) and use storage — so index what you search and join on, not everything.
Primary keys are indexed automatically; add indexes to columns in WHERE and JOIN clauses.
A query plan (EXPLAIN) shows whether an index is being used — the first tool for tuning slow queries.

This directly supports the JOINs you'll write most.

Interview Intelligence

How this topic actually shows up in interviews — and how to demonstrate you understand it.

Why employers ask about this

Query performance separates capable SQL users from those who only write correct queries.

Technical questions

How does an index speed up a query, and what's the cost?+

It avoids a full scan for faster reads, at the cost of slower writes and extra storage.

Behavioural questions

Describe improving something's performance.+

Show measuring first (query plan), making a targeted change and confirming the gain.

Real-world scenarios

“A report query is suddenly very slow.”+

Expected answer: Run EXPLAIN, check for a missing index on the filtered/joined columns, and add one if justified.

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Employability Intelligence

Where this knowledge takes you — the jobs, skills and certifications it feeds into.

Relevant roles

Database AdministratorData EngineerBackend Developer

Skills you're proving

IndexingQuery plansTuning

Recommended certifications

Microsoft DP-900 (Data Fundamentals)Oracle / SQL certifications

Career progression

Databases & SQL → DBA/data engineering roles.

What employers expect

That you can design a sensible schema and write correct, efficient SQL.

Frequently asked questions

What is a database index?

A data structure that lets the database find rows quickly without scanning the whole table.

Why not index every column?

Indexes slow down writes and use storage; index the columns you filter and join on.

How do you diagnose a slow query?

Use EXPLAIN to read the query plan and check whether an index is being used.

Practise what you've learned

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