Deep understanding of ClickHouse internals: MergeTree family engines, materialized views, projections, dictionaries, sharding, replication, and query optimization; index-aware table design (ORDER BY, primary key, data-skipping indexes) and tuning; production troubleshooting - diagnosing errors such as excessive parts ("too many parts"), replication lag, and related cluster issues; performance analysis under concurrency and load, not only optimizing isolated queries. 5+ years of backend or data engineering experience, with 2+ years of hands-on ClickHouse in production - you ve designed schemas, tuned queries, and operated clusters under real load, not just ran tutorials.