Troubleshoot Slow Query Plans | Dataford Interview Questions

How would you troubleshoot a slow query using execution plans?

Explain a repeatable PostgreSQL workflow for identifying the bottleneck, validating whether indexes are useful, and separating planner estimates from actual execution behavior. Address join strategies, row estimates, sorting, aggregation, buffer activity, and statistics.

Output

A concise diagnosis workflow and the plan evidence to inspect
Recommended validation steps and possible corrective actions
A PostgreSQL query that can be analyzed with EXPLAIN (ANALYZE, BUFFERS)

Schema

customers

Column	Type	Description
customer_idPK	INT	Customer identifier
customer_name	VARCHAR(100)	Customer display name

orders

Column	Type	Description
order_idPK	INT	Order identifier
customer_id	INT	Customer who placed the order
created_at	DATE	Order creation date

order_items

Column	Type	Description
order_item_idPK	INT	Order item identifier
order_id	INT	Order containing the item
quantity	INT	Number of units
unit_price	NUMERIC(10,2)	Price per unit

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Problem