Optimizing Slow SQL and ETL | Dataford Interview Questions

What is your approach to optimizing slow-running SQL queries or ETL jobs?

For this hands-on exercise, write PostgreSQL SQL that returns premium totals for active policies during 2025. Your solution should demonstrate an optimization-minded approach, including early filtering and appropriate handling of policies without qualifying payments.

Output

One row per active policy issued before 2026-01-01.
Columns: policy_id, customer_name, product_name, total_successful_premium, and latest_successful_payment.
Include policies with no successful 2025 payments, showing zero and NULL respectively. Order by policy_id ascending.

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(120)	Customer full name
city	VARCHAR(80)	Customer city

policies

Column	Type	Description
policy_idPK	INT	Unique policy identifier
customer_id	INT	Customer who owns the policy
product_name	VARCHAR(100)	Insurance product name
status	VARCHAR(20)	Current policy status
issue_date	DATE	Date the policy was issued

premium_payments

Column	Type	Description
payment_idPK	INT	Unique payment identifier
policy_id	INT	Policy associated with the payment
payment_date	DATE	Date the payment was recorded
amount	DECIMAL(12,2)	Payment amount
payment_status	VARCHAR(20)	Payment processing status

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

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Problem