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QueryPlan

Understand SQL, one query at a time.

Write SQL in your browser and see how the database finds your rows.

Explore learning paths Try the free query plan visualizer
The same query before and after an index
SELECT id, created_at, total_cents
FROM orders
WHERE customer_id = 42
ORDER BY created_at DESC;

Before the index

  • SCAN ordersReads every row in the table.
  • USE TEMP B-TREE FOR ORDER BYSorts the rows in a temporary table.

After CREATE INDEX idx_orders_customer_created

  • SEARCH orders USING INDEX idx_orders_customer_created (customer_id=?)Finds the matching rows through the index, already in order.
  • 5 modules
  • 50 exercises
  • Module 1 is free
  • $99 for lifetime access
  • Runs in your browser

Choose where to start

SQL foundations

Four free lessons on tables, columns, filters and sorting.

Start SQL foundations

SQL performance

50 exercises on indexes, joins and query plans. The first module is free.

Start the free module

What the performance course covers

Plan on about 9 to 21 hours.

  1. Reading queriesFree10 exercises
  2. How queries run8 exercises
  3. Indexes8 exercises
  4. Joins12 exercises
  5. Sorting and grouping12 exercises

Modules 2 to 5 cost $99 once, for lifetime access. Module 1 is free.

See Modules 2 to 5

All sales are final except where required by applicable law. Terms.

Frequently asked questions

Can I start without SQL experience?

Yes. The free foundations path introduces tables, SELECT, filtering, and sorting in four guided lessons. It is a starting point; the SQL performance course assumes you can already write basic queries.

Is QueryPlan video-based?

No. You learn by writing SQL, reading the query plan, and revising your query.

Why SQLite instead of PostgreSQL or MySQL?

SQLite can run real queries in your browser. The main questions transfer to other databases: Did the query scan a table, search an index, repeat a broad join, or create a temporary sort? PostgreSQL and MySQL use different plan terms and may choose different work.

Who is this for?

Start with the free foundations path if you are new to SQL. If you know the basics, the performance course helps you understand indexes, joins, and execution plans.