writing-clickhouse-queries

★ 35,336

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.

🧰 Not standalone. This skill ships with posthog/posthog and only works together with that tool — install the tool first, then add this skill.

This is the playbook your agent receives when the skill activates — you don't need to read it to use the skill, but it's here to audit before installing.

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.