Optimizing Slow BigQuery Queries | Dataford Interview Questions

Walk me through your process for optimizing a slow-running query in BigQuery.

For the hands-on portion, use the supplied PostgreSQL tables and produce the optimized result query. Discuss how you would inspect the execution plan, reduce scanned rows, validate correctness, and confirm an improvement.

Output

One row per active query and run date, with query_name, owner_team, run_date, run_count, avg_duration_ms, and total_bytes_processed.
Include only successful runs from January 2026 whose duration is at least 1,000 milliseconds, ordered by query_name, then run_date.

Schema

query_runs

Column	Type	Description
run_idPK	INT	Unique query execution identifier
query_id	INT	Referenced query definition
started_at	TIMESTAMP	Execution start timestamp
duration_ms	INT	Execution duration in milliseconds
bytes_processed	BIGINT	Bytes processed by the execution
status	VARCHAR(20)	Execution status

query_catalog

Column	Type	Description
query_idPK	INT	Unique query definition identifier
query_name	VARCHAR(100)	Human-readable query name
owner_team	VARCHAR(80)	Team responsible for the query
is_active	BOOLEAN	Whether the query definition is active

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

Run and submit as often as you like. When you're ready, talk me through your approach or go straight to the code.

Problem