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Writing Clickhouse Queries

posthog/writing-clickhouse-queries

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.

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on the repository, not the skill itself

Install

one command, takes just this skill from the repository 
npx skills add https://github.com/PostHog/posthog --skill writing-clickhouse-queries

The instruction itself

2 sections, as written by the author

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.

How to use it

Copy the folder

Take posthog/writing-clickhouse-queries from the repository into ~/.claude/skills for personal use, or into .claude/skills inside a project.

Check the name does not clash

The agent identifies a skill by the name field in its header. Two skills with the same name cannot sit side by side — one of them will be ignored.