An Index for a Slow Search

The orders page finds one customer's orders with WHERE customer_id = ?. How does SQLite find them? You can ask it: put EXPLAIN QUERY PLAN in front of a query, and SQLite doesn't run it; it answers rows that describe how it would run it. Each row has a detail column, a sentence about one step, so in TypeScript you describe the rows as { detail: string }[].

A plan that says SCAN means SQLite reads every row of the table and keeps the matches: quick for 7 orders, slow for 7 million. An index is a sorted list of one column's values, each pointing at its rows, like the index at the back of a book. With one, the plan says SEARCH: SQLite jumps straight to the matching rows. Here is an index on another table and column, for searches by category:

db.exec("CREATE INDEX IF NOT EXISTS idx_products_category ON products (category)");

db.exec runs SQL that answers no rows, and IF NOT EXISTS lets the app run this line every time it starts without failing. An index changes how rows are found, never which rows: every answer stays exactly the same. Your code must give these answers:

GET /debug/plan  →  200  {"plan": "SEARCH orders USING INDEX idx_orders_customer_id (customer_id=?)"}
GET /customers/3/orders  →  200  [{"id": 4, …, "status": "shipped"}, {"id": 7, …, "status": "paid"}]
GET /customers/5/orders  →  200  []
GET /debug/queries  →  200  {"queries": 4}

Asking for a plan is not a query the shop runs, so it goes through db.prepare, not runQuery, and isn't counted: the 4 queries are 3 for customer 3's orders and 1 for customer 5's.

Your task: add a route for GET /debug/plan that runs the orders page's query with EXPLAIN QUERY PLAN in front, for customer 1, keeps the rows as { detail: string }[], and answers the first row's detail as { plan: ... }. It will say SCAN orders. Then, below const db, use db.exec to create the index idx_orders_customer_id on orders (customer_id), and the plan becomes a SEARCH.

Assignment
Follow the Coding Tutorial to show the plan of the orders query at GET /debug/plan, then add the index that turns its SCAN into a SEARCH.

Hint
rows[0].detail is the first row's sentence. .all(1) puts 1 in place of the ?.