Optimize Slow SQL on Large DB | Dataford Interview Questions

How do you optimize a SQL query that is running slowly on a very large database?

Use the provided PostgreSQL schema as the working example. Produce the result for completed orders from the first quarter of 2025, and explain how you would investigate and improve performance at scale.

Output

One row per customer with qualifying orders.
Columns: customer_id, customer_name, region, order_count, and total_revenue.
Include only completed orders dated from 2025-01-01 through 2025-03-31, with revenue above 500.
Sort by total_revenue descending, then customer_id ascending.

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer name
region	VARCHAR(50)	Customer region

orders

Column	Type	Description
order_idPK	INT	Unique order identifier
customer_id	INT	Customer who placed the order
order_date	DATE	Date 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	Order containing the item
quantity	INT	Number of units ordered
unit_price	NUMERIC(10,2)	Price per unit

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Problem