Optimizing Slow SQL Queries | Dataford Interview Questions

What is your approach to optimizing slow SQL queries?

Using the provided PostgreSQL schema, produce an equivalent query for the required result. Explain how you would inspect the execution plan, reduce unnecessary work, and select appropriate indexes without changing the result.

Output

One row per customer whose completed January 2025 revenue exceeds 100.
Columns: customer_name, order_count, and total_spent.
Sort by total_spent descending, then customer_id ascending for ties.

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer display name
email	VARCHAR(255)	Customer email address

orders

Column	Type	Description
order_idPK	INT	Unique order identifier
customer_id	INT	References customers.customer_id
order_date	DATE	Date on which the order was placed
status	VARCHAR(20)	Order processing status

order_items

Column	Type	Description
order_item_idPK	INT	Unique order line identifier
order_id	INT	References orders.order_id
product_name	VARCHAR(100)	Product description
quantity	INT	Number of units in the order line
unit_price	DECIMAL(10,2)	Price per unit

Your question is Optimizing Slow SQL Queries. Start with the requirements and the three tables in the Question tab.

Run and submit as often as you like. When you're ready, talk me through your approach or go straight to the code.

Problem