SQL Performance Optimization Strategies | Dataford Interview Questions

What strategies would you use to optimize a SQL query for performance?

Explain your approach and provide a PostgreSQL query that returns customer totals for completed orders from January 1 through March 31, 2025. Include the reasoning behind query structure, filtering, indexing, and plan inspection.

Output

One row per qualifying customer with customer_id, customer_name, completed_orders, and total_spend.
Include customers with at least two completed orders in the date range.
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
email	VARCHAR(255)	Customer email address

orders

Column	Type	Description
order_idPK	INT	Unique order identifier
customer_id	INT	Customer who placed the order
status	VARCHAR(20)	Order lifecycle status
order_date	DATE	Date the order was placed

order_items

Column	Type	Description
order_item_idPK	INT	Unique order-line identifier
order_id	INT	Order containing the item
sku	VARCHAR(30)	Item stock keeping unit
quantity	INT	Quantity purchased
unit_price	NUMERIC(10,2)	Price per item

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