Step 4 of 7
Indexes and ACID transactions
An index is like a book's index: it lets the database jump to rows instead of scanning the whole table. A transaction groups several changes so they all succeed or all fail (ACID: atomic, consistent, isolated, durable).
Why it matters when you build with AI: Missing indexes are why queries get slow as you grow. Missing transactions are why a payment gets recorded but the order doesn't.
Quick check
Payment recorded but order missing after a crash. What was missing?
A transaction makes related writes all succeed or all fail.