Optimize a Slow SQL Query | Dataford Interview Questions

How would you optimize a slow-running SQL query?

Given the supplied PostgreSQL schema and data, rewrite the query so it returns the same business result while reducing unnecessary scans and aggregation work. The result should include active loans with payments during the specified date range and more than 1,000 in total payments.

Output

One row per qualifying active loan
Columns: loan_id, borrower_name, and total_paid
Include payments from 2025-01-01 through 2025-02-28
Sort by total_paid descending, then loan_id ascending

Schema

borrowers

Column	Type	Description
borrower_idPK	INT	Unique borrower identifier
borrower_name	VARCHAR(100)	Borrower display name

loans

Column	Type	Description
loan_idPK	INT	Unique loan identifier
borrower_id	INT	Borrower associated with the loan
status	VARCHAR(20)	Current loan status
original_principal	DECIMAL(12,2)	Original principal balance

loan_payments

Column	Type	Description
payment_idPK	INT	Unique payment identifier
loan_id	INT	Loan associated with the payment
payment_date	DATE	Date the payment was recorded
amount	DECIMAL(12,2)	Payment amount

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Problem