Writing ClickHouse queries for new products

If you're optimizing an existing query rather than writing a new one, this is the wrong skill. Use `/optimizing-clickhouse-and-hogql-queries` instead. That skill covers layer triage, smell scanning (FROM ... FINAL, JSONExtract over properties, missing skip indexes, self-joins, CTE blow-up), measurement on the Test Cluster, and applying the fix at the right layer.

Read `docs/published/handbook/engineering/databases/clickhouse-queries-new-products.md` for the authoritative guide on writing new queries.

Then pull in whichever related docs the task touches:

• `hogql-python.md` for HogQL in Python
• `materialized-columns.md`
• `query-performance-optimization.md`

When to use

• Writing or reviewing a QueryRunner subclass in posthog/hogql_queries/ or products/*/backend/
• Adding a new ClickHouse table or ALTER for a product (posthog/clickhouse/migrations/)
• Choosing a row ID format for a new table
• Adding or removing materialized columns, skip indexes, or projections

For investigating an existing slow query, debugging a system.query_log row, or reviewing a proposed HogQL printer change for performance, use `/optimizing-clickhouse-and-hogql-queries`.

Not the right skill for: customer-facing ad-hoc HogQL via Max / posthog:execute-sql, use query-examples for that. For migration mechanics (node roles, engines, replication), use clickhouse-migrations.