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](../optimizing-clickhouse-and-hogql-queries/SKILL.md) 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](../../../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](../../../docs/published/handbook/engineering/databases/hogql-python.md) for HogQL in Python
• [materialized-columns.md](../../../docs/published/handbook/engineering/databases/materialized-columns.md)
• [query-performance-optimization.md](../../../docs/published/handbook/engineering/databases/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](../optimizing-clickhouse-and-hogql-queries/SKILL.md).

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.