Optimizing Slow SQL Queries | Dataford Interview Questions

How would you optimize a SQL query that is running slowly?

Use the customers and orders tables to produce the requested result. The answer should preserve the required filters and aggregation while being suitable for performance analysis in PostgreSQL.

Output

One row per customer with qualifying completed orders in January 2025.
Columns: customer_id, customer_name, completed_order_count, and total_completed_amount.
Include only customers whose completed January total is at least 100.00. Order by total amount descending, then customer ID ascending.

Schema

customers

Column	Type	Description
customer_idPK	VARCHAR(20)	Unique customer identifier
customer_name	VARCHAR(100)	Customer display name

orders

Column	Type	Description
order_idPK	INT	Unique order identifier
customer_id	VARCHAR(20)	Customer associated with the order
order_date	DATE	Order creation date
status	VARCHAR(40)	Current order status
amount	NUMERIC(12,2)	Order amount

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

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Problem