Lesson 19 — Avoiding N+1

The N+1 query problem

If you show 100 orders and look up each order's user with a separate database query, that is 1 query for the orders + 100 for users = N+1 queries, and the page gets slow. The fix is to fetch all needed users once and keep them in a dict. Django does this with select_related / prefetch_related.

Here a counter shows how many "queries" we make.

Note: the exercise runs in your browser without Django installed, so it is a small plain-Python simulation of what Django does for you.

Why it matters: N+1 is the most common reason a Django page is slow — and the fix is a few lines.