| name | writing-clickhouse-queries |
| description | Guide for writing performant ClickHouse queries in PostHog product code. Use when writing HogQL query runners, designing a ClickHouse table for a new product, adding materialized columns or skip indexes, or choosing a row ID format. For optimizing an existing query that is already too slow, use `/optimizing-clickhouse-and-hogql-queries` instead. |
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:
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.
