Optimize Slow SQL in Data Warehouse | Dataford Interview Questions

How would you optimize a slow-running SQL query within a large-scale data warehouse?

Use the supplied warehouse tables to demonstrate an optimized query that returns qualifying customers and their completed order totals for the first half of 2025. Keep the result deterministic and suitable for evaluating query performance.

Output

One row per qualifying customer, with customer_id, customer_name, and total_spend
Include only customers with at least 1000 in completed orders during the period
Sort by total_spend descending, then customer_id ascending

Schema

customers

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

orders

Column	Type	Description
order_idPK	INT	Unique order identifier
customer_id	INT	Customer associated with the order
order_total	NUMERIC(12,2)	Total monetary value of the order
order_status	VARCHAR(20)	Order processing status
order_date	DATE	Date the order was placed

Your question is Optimize Slow SQL in Data Warehouse. Start with the requirements and the two 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