Fixing Slow SQL Queries | Dataford Interview Questions

How do you optimize a slow-running SQL query at Citi?

Using the accounts and transactions tables, write an optimized PostgreSQL query that returns active accounts whose completed transactions during Q1 2025 total at least 1,000. The query should avoid unnecessary rows before aggregation and produce a deterministic result.

Output

One row per qualifying active account
Columns: account_id, account_name, q1_completed_amount, completed_transaction_count
Include only completed transactions from 2025-01-01 through 2025-03-31 and accounts meeting the amount threshold
Order by q1_completed_amount descending, then account_id ascending

Schema

accounts

Column	Type	Description
account_idPK	INT	Unique account identifier
account_name	VARCHAR(100)	Account holder name
account_status	VARCHAR(20)	Current account status
account_type	VARCHAR(30)	Account product type

transactions

Column	Type	Description
transaction_idPK	INT	Unique transaction identifier
account_id	INT	Related account identifier
transaction_date	DATE	Transaction posting date
status	VARCHAR(20)	Transaction processing status
amount	DECIMAL(12,2)	Transaction amount

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Problem