Debugging a Slow SQL Query | Dataford Interview Questions

How would you approach debugging a slow-performing SQL query for Zomato analytics?

Focus on how you would identify the bottleneck, validate the query plan, and decide what to change first. Keep the answer practical and specific to PostgreSQL behavior.

Output

A concise debugging approach for a slow SQL query
The key checks you would perform in order
The main query changes you would consider first

Schema

orders

Column	Type	Description
order_idPK	BIGINT	Unique order identifier
restaurant_id	BIGINT	Restaurant that received the order
customer_id	BIGINT	Customer who placed the order
order_status	VARCHAR(20)	Order state such as delivered, cancelled, or pending
order_created_at	TIMESTAMP	Order creation timestamp
order_value	NUMERIC(10,2)	Order amount

restaurants

Column	Type	Description
restaurant_idPK	BIGINT	Unique restaurant identifier
city_name	VARCHAR(50)	City where the restaurant operates
is_active	BOOLEAN	Whether the restaurant is currently active

delivery_events

Column	Type	Description
event_idPK	BIGINT	Unique delivery event identifier
order_id	BIGINT	Order associated with the event
event_type	VARCHAR(30)	Event type such as picked_up or delivered
event_time	TIMESTAMP	Timestamp of the delivery event

Your question is Debugging a Slow SQL Query. Start with the requirements and the three tables in the Question tab.

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Problem