clickhouse

name	clickhouse
description	MUST USE when reviewing ClickHouse schemas, queries, or configurations. Contains 28 rules that MUST be checked before providing recommendations. Always read relevant rule files and cite specific rules in responses.

ClickHouse Best Practices

Guidance for ClickHouse covering schema design, query optimization, and data ingestion. Contains 28 atomic rules across 3 categories (schema, query, insert), prioritized by impact. Extended with 14 reference files covering cluster management, backups, monitoring, and integrations.

Official docs: ClickHouse Best Practices

Official docs: ClickHouse Best Practices

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⚠️ Security Considerations

Credential Placeholders

Example credentials in documentation (password123, AKIAIOSFODNN7EXAMPLE) are placeholders only. Never use these in production. Use proper secret management:

• Environment variables
• Secret managers (AWS Secrets Manager, HashiCorp Vault, etc.)
• Kubernetes secrets (for K8s deployments)
• ClickHouse named collections with external configuration

Installation & Operations

For installation and operational procedures:

• Follow official documentation links provided in reference files
• Prefer package managers (apt, yum, helm) over direct downloads
• Use versioned artifacts instead of latest in production
• Test procedures in non-production environments first

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IMPORTANT: How to Apply This Skill

Before answering ClickHouse questions, follow this priority order:

1. Check for applicable rules in the rules/ directory
2. If rules exist: Apply them and cite them in your response using "Per rule-name..."
3. If no rule exists: Check references/ for deeper topic coverage
4. If neither covers it: Use general ClickHouse knowledge or search documentation
5. Always cite your source: rule name, reference file, or URL

Why rules take priority: ClickHouse has specific behaviors (columnar storage, sparse indexes, merge tree mechanics) where general database intuition can be misleading. The rules encode validated, ClickHouse-specific guidance.

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Review Procedures

For Schema Reviews (CREATE TABLE, ALTER TABLE)

Read these rule files in order:

1. rules/schema-pk-plan-before-creation.md — ORDER BY is immutable
2. rules/schema-pk-cardinality-order.md — Column ordering in keys
3. rules/schema-pk-prioritize-filters.md — Filter column inclusion
4. rules/schema-pk-filter-on-orderby.md — Query filter alignment
5. rules/schema-types-native-types.md — Proper type selection
6. rules/schema-types-minimize-bitwidth.md — Numeric type sizing
7. rules/schema-types-lowcardinality.md — LowCardinality usage
8. rules/schema-types-avoid-nullable.md — Nullable vs DEFAULT
9. rules/schema-types-enum.md — Enum for finite value sets
10. rules/schema-partition-low-cardinality.md — Partition count limits
11. rules/schema-partition-lifecycle.md — Partitioning purpose
12. rules/schema-partition-query-tradeoffs.md — Partition pruning trade-offs
13. rules/schema-partition-start-without.md — Start without partitioning
14. rules/schema-json-when-to-use.md — JSON type usage

Check for:

• PRIMARY KEY / ORDER BY column order (low-to-high cardinality)
• Data types match actual data ranges
• LowCardinality applied to appropriate string columns
• Partition key cardinality bounded (100-1,000 values)
• ReplacingMergeTree has version column if used

For Query Reviews (SELECT, JOIN, aggregations)

Read these rule files:

1. rules/query-join-choose-algorithm.md — Algorithm selection
2. rules/query-join-use-any.md — ANY vs regular JOIN
3. rules/query-join-filter-before.md — Pre-join filtering
4. rules/query-join-consider-alternatives.md — Dictionaries/denormalization
5. rules/query-join-null-handling.md — join_use_nulls setting
6. rules/query-index-skipping-indices.md — Secondary index usage
7. rules/query-mv-incremental.md — Incremental materialized views
8. rules/query-mv-refreshable.md — Refreshable materialized views

Check for:

• Filters use ORDER BY prefix columns
• JOINs filter tables before joining (not after)
• Correct JOIN algorithm for table sizes
• Skipping indices for non-ORDER BY filter columns

For Insert Strategy Reviews (data ingestion, updates, deletes)

Read these rule files:

1. rules/insert-batch-size.md — Batch sizing requirements
2. rules/insert-async-small-batches.md — Async insert usage
3. rules/insert-format-native.md — Native format for performance
4. rules/insert-mutation-avoid-update.md — UPDATE alternatives
5. rules/insert-mutation-avoid-delete.md — DELETE alternatives
6. rules/insert-optimize-avoid-final.md — OPTIMIZE TABLE risks

Check for:

• Batch size 10K-100K rows per INSERT
• No ALTER TABLE UPDATE for frequent changes
• ReplacingMergeTree or CollapsingMergeTree for update patterns
• Async inserts enabled for high-frequency small batches

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Output Format

Structure review responses as follows:

## Rules Checked
- `rule-name-1` — Compliant / Violation found
- `rule-name-2` — Compliant / Violation found
...

## Findings

### Violations
- **`rule-name`**: Description of the issue
  - Current: [what the code does]
  - Required: [what it should do]
  - Fix: [specific correction]

### Compliant
- `rule-name`: Brief note on why it's correct

## Recommendations
[Prioritized list of changes, citing rules]

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Rule Categories by Priority

Priority	Category	Impact	Prefix	Count
1	Primary Key Selection	CRITICAL	schema-pk-	4
2	Data Type Selection	CRITICAL	schema-types-	5
3	JOIN Optimization	CRITICAL	query-join-	5
4	Insert Batching	CRITICAL	insert-batch-	1
5	Mutation Avoidance	CRITICAL	insert-mutation-	2
6	Partitioning Strategy	HIGH	schema-partition-	4
7	Skipping Indices	HIGH	query-index-	1
8	Materialized Views	HIGH	query-mv-	2
9	Async Inserts	HIGH	insert-async-	2
10	OPTIMIZE Avoidance	HIGH	insert-optimize-	1
11	JSON Usage	MEDIUM	schema-json-	1

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Quick Reference

Schema Design — Primary Key (CRITICAL)

• schema-pk-plan-before-creation — Plan ORDER BY before table creation (immutable)
• schema-pk-cardinality-order — Order columns low-to-high cardinality
• schema-pk-prioritize-filters — Include frequently filtered columns
• schema-pk-filter-on-orderby — Query filters must use ORDER BY prefix

Schema Design — Data Types (CRITICAL)

• schema-types-native-types — Use native types, not String for everything
• schema-types-minimize-bitwidth — Use smallest numeric type that fits
• schema-types-lowcardinality — LowCardinality for <10K unique strings
• schema-types-enum — Enum for finite value sets with validation
• schema-types-avoid-nullable — Avoid Nullable; use DEFAULT instead

Schema Design — Partitioning (HIGH)

• schema-partition-low-cardinality — Keep partition count 100-1,000
• schema-partition-lifecycle — Use partitioning for data lifecycle, not queries
• schema-partition-query-tradeoffs — Understand partition pruning trade-offs
• schema-partition-start-without — Consider starting without partitioning

Schema Design — JSON (MEDIUM)

• schema-json-when-to-use — JSON for dynamic schemas; typed columns for known

Query Optimization — JOINs (CRITICAL)

• query-join-choose-algorithm — Select algorithm based on table sizes
• query-join-use-any — ANY JOIN when only one match needed
• query-join-filter-before — Filter tables before joining
• query-join-consider-alternatives — Dictionaries/denormalization vs JOIN
• query-join-null-handling — join_use_nulls=0 for default values

Query Optimization — Indices (HIGH)

• query-index-skipping-indices — Skipping indices for non-ORDER BY filters

Query Optimization — Materialized Views (HIGH)

• query-mv-incremental — Incremental MVs for real-time aggregations
• query-mv-refreshable — Refreshable MVs for complex joins

Insert Strategy — Batching (CRITICAL)

• insert-batch-size — Batch 10K-100K rows per INSERT

Insert Strategy — Async (HIGH)

• insert-async-small-batches — Async inserts for high-frequency small batches
• insert-format-native — Native format for best performance

Insert Strategy — Mutations (CRITICAL)

• insert-mutation-avoid-update — ReplacingMergeTree instead of ALTER UPDATE
• insert-mutation-avoid-delete — Lightweight DELETE or DROP PARTITION

Insert Strategy — Optimization (HIGH)

• insert-optimize-avoid-final — Let background merges work

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Quick Decision Guides

Which Table Engine?

Need to store data?
├── < 1M rows, dimension → Memory
└── ≥ 1M rows → MergeTree family
    ├── Deduplication? → ReplacingMergeTree(version)
    ├── Changelog? → CollapsingMergeTree(sign)
    ├── Pre-aggregation? → AggregatingMergeTree()
    ├── Replication? → ReplicatedMergeTree(...)
    └── Default → MergeTree()

See references/table-engines.md for complete reference.

Common Issues & Quick Fixes

Issue	Quick Fix
Too many parts	OPTIMIZE TABLE table FINAL (see insert-optimize-avoid-final)
Slow query	EXPLAIN SELECT ... to check index usage
Mutation stuck	Check system.mutations, consider alternatives per insert-mutation-avoid-update
Replication lag	Check system.replication_queue, ZooKeeper
OOM on query	Increase max_memory_usage, optimize query

See references/debugging.md for detailed troubleshooting.

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Deep Reference Files

For topics beyond the 28 rules, see the references/ directory:

Schema & Table Design

• references/core-concepts.md — Architecture, data model, internals
• references/schema-design.md — Database engines, migrations, version control
• references/table-design.md — ORDER BY, partitioning, column selection
• references/table-engines.md — Complete MergeTree family reference

Query & Performance

• references/sql-reference.md — Complete SQL dialect, data types
• references/query-optimization.md — EXPLAIN, JOINs, projections, skip indexes
• references/advanced-features.md — Materialized views, mutations, TTL, dictionaries

Operations & Cluster

• references/debugging.md — Query debugging, merges, mutations, replication
• references/cluster-management.md — Distributed tables, replication, sharding
• references/backup-restore.md — Backup strategies, disaster recovery
• references/monitoring.md — Query monitoring, health checks, system queries

Integration & Best Practices

• references/integrations.md — Kafka, S3, PostgreSQL, MySQL, BI tools
• references/best-practices.md — Complete checklist and anti-patterns
• references/external.md — Altinity KB links, official docs
• references/system-queries.md — Ready-to-use queries for operations

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Version: 1.3.0 Rules: Synced with ClickHouse/agent-skills (Apache-2.0) References: Altinity Knowledge Base (200+ articles) + ClickHouse Official Docs