Mitigate Data Skew in Joins | Dataford Interview Questions

What is data skew, and what strategies would you use to mitigate it in a large-scale join operation?

Using the provided PostgreSQL tables, demonstrate one SQL-based mitigation by assigning four deterministic salt buckets to fact rows and replicating dimension rows across those buckets. Flag join keys with at least three fact rows.

Output

Return one row per matched event with event_id, customer_id, event_type, segment, salt_bucket, key_event_count, and is_skewed.
Include only matched events, ordered by customer_id and event_id.

Schema

customer_events

Column	Type	Description
event_idPK	INT	Unique event identifier
customer_id	VARCHAR(30)	Customer associated with the event
event_type	VARCHAR(20)	Type of customer event
event_date	DATE	Date the event occurred

customer_accounts

Column	Type	Description
customer_idPK	VARCHAR(30)	Unique customer identifier
segment	VARCHAR(20)	Customer segment
region	VARCHAR(20)	Customer operating region

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