What is clickhouse-migrations?

A guide to writing ClickHouse database migrations, which are controlled changes to tables, views, and related database objects.

Use it when creating or changing ClickHouse migrations, especially for choosing the owning cluster and handling sharded or replicated objects.

It helps migrations run on the correct database cluster and use valid roles and operations, avoiding failures that can stop migration jobs.

How PostHog/posthog-foss configures Claude Code and Codex

Reuse

Borrowing it

Nothing to install: this file belongs to PostHog/posthog-foss. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file

curl -O https://raw.githubusercontent.com/PostHog/posthog-foss/master/.agents/skills/clickhouse-migrations/SKILL.md

Clone the repo

git clone --depth 1 https://github.com/PostHog/posthog-foss

Made for: Claude Code, Codex.

Wrote this? Show the measurements

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README.md

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Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

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Per session 21 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 2,080 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe. Third-party audits

NVIDIA SkillSpector pass 7 Sept 2026

How audits are shown

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00021	$0.02080
Opus 5	$0.00010	$0.01040
Sonnet 5	$0.00004	$0.00416
Haiku 4.5	$0.00002	$0.00208

Measured 10d ago against content hash 3f858854398b, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-10, from the pricing page.

Security

Grade A, and why

clickhouse-migrations scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 10d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

.agents/skills/clickhouse-migrations/SKILL.md · 143 lines

How it starts

The opening of the file, as written. The whole thing — 143 lines — stays where its author put it; the contents beside it link to each section on GitHub.

ClickHouse Migrations

Read posthog/clickhouse/migrations/AGENTS.md for comprehensive patterns, cluster setup, examples, and ingestion layer details.

Quick reference

Migration structure

operations = [
    run_sql_with_exceptions(
        SQL_FUNCTION(),
        node_roles=[...],
        sharded=False,  # True for sharded tables
        is_alter_on_replicated_table=False  # True for ALTER on replicated tables
    ),
]

Node roles (choose by the cluster that owns the object)

Only the members of NodeRole in posthog/clickhouse/client/connection.py are valid: ALL, DATA, INGESTION_EVENTS, INGESTION_SMALL, INGESTION_MEDIUM, ENDPOINTS, LOGS, AI_EVENTS, AUX, BATCH_EXPORTS, OPS, SESSIONS. A name that is not in that enum fails at import, which aborts migration discovery and takes every job that runs migrations down with it.

Pick the role by which cluster owns the object, not by its type alone:

[NodeRole.DATA]: anything on the main cluster — sharded tables, non-sharded replicated tables, distributed read tables, views, dictionaries. The default, and right for most migrations.
[NodeRole.INGESTION_SMALL]: writable tables, Kafka tables, materialized views on the ingestion layer
[NodeRole.OPS], [NodeRole.LOGS], [NodeRole.AUX], [NodeRole.AI_EVENTS], [NodeRole.SESSIONS], [NodeRole.BATCH_EXPORTS]: objects that live on a satellite cluster. A table for one of those on DATA lands on the wrong nodes and leaves the intended cluster without it, so check where the object is read and written before defaulting to DATA. Dev runs the same satellite clusters as US/EU prod — never branch on CLOUD_DEPLOYMENT to give dev a different layout.
[NodeRole.ALL]: rarely used

Table engines quick reference

MergeTree engines:

AggregatingMergeTree(table, replication_scheme=ReplicationScheme.SHARDED) for sharded tables
ReplacingMergeTree(table, replication_scheme=ReplicationScheme.REPLICATED) for non-sharded
Other variants: CollapsingMergeTree, ReplacingMergeTreeDeleted

Read the full file on GitHub · 143 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

10d ago First seen · 143 lines · 21 tokens per session scan A 3f858854398b

Subscribe to this mod's changes

clickhouse-migrations is a skill published in the GitHub repository PostHog/posthog-foss (715 stars, last pushed today), licensed MIT. It adds 21 tokens to every session and 2,080 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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posthog-foss: Skill for Claude Code