Optimizing Slow SQL Queries | Dataford Interview Questions

How would you optimize a slow-running SQL query involving millions of rows?

Use the supplied PostgreSQL tables to return successful transactions during January 2025. The solution should be suitable for a large transaction table and preserve customers whose qualifying transactions have a NULL amount.

Output

One row per customer with at least one qualifying transaction.
Columns: customer_id, customer_name, successful_recharge_count, and total_amount.
Sort by total_amount descending, then customer_id ascending.

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer display name
segment	VARCHAR(30)	Optional customer segment

recharge_transactions

Column	Type	Description
transaction_idPK	BIGINT	Unique recharge transaction identifier
customer_id	INT	Customer associated with the transaction
amount	NUMERIC(12,2)	Recharge amount
transaction_ts	TIMESTAMP	Transaction timestamp
status	VARCHAR(20)	Transaction processing status
channel	VARCHAR(20)	Recharge channel

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

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Problem