ClickHouse Best Practices

Comprehensive guidance for ClickHouse covering schema design, query optimization, and data ingestion. Contains 28 rules across 3 main categories (schema, query, insert), prioritized by impact.

Official docs: ClickHouse Best Practices

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: Use the LLM's ClickHouse knowledge or search documentation
4. •If uncertain: Use web search for current best practices
5. •Always cite your source: rule name, "general ClickHouse guidance", 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.

For Formal Reviews

When performing a formal review of schemas, queries, or data ingestion:

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-types-native-types.md - Proper type selection
5. •rules/schema-types-minimize-bitwidth.md - Numeric type sizing
6. •rules/schema-types-lowcardinality.md - LowCardinality usage
7. •rules/schema-types-avoid-nullable.md - Nullable vs DEFAULT
8. •rules/schema-partition-low-cardinality.md - Partition count limits
9. •rules/schema-partition-lifecycle.md - Partitioning purpose

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-filter-before.md - Pre-join filtering
3. •rules/query-join-use-any.md - ANY vs regular JOIN
4. •rules/query-index-skipping-indices.md - Secondary index usage
5. •rules/schema-pk-filter-on-orderby.md - Filter alignment with ORDER BY

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-mutation-avoid-update.md - UPDATE alternatives
3. •rules/insert-mutation-avoid-delete.md - DELETE alternatives
4. •rules/insert-async-small-batches.md - Async insert usage
5. •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

Output Format

Structure your response 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]

Rule Categories by Priority

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

When to Apply

This skill activates when you encounter:

• •CREATE TABLE statements
• •ALTER TABLE modifications
• •ORDER BY or PRIMARY KEY discussions
• •Data type selection questions
• •Slow query troubleshooting
• •JOIN optimization requests
• •Data ingestion pipeline design
• •Update/delete strategy questions
• •ReplacingMergeTree or other specialized engine usage
• •Partitioning strategy decisions

Rule File Structure

Each rule file in rules/ contains:

• •YAML frontmatter: title, impact level, tags
• •Brief explanation: Why this rule matters
• •Incorrect example: Anti-pattern with explanation
• •Correct example: Best practice with explanation
• •Additional context: Trade-offs, when to apply, references

Full Compiled Document

For the complete guide with all rules expanded inline: AGENTS.md

Use AGENTS.md when you need to check multiple rules quickly without reading individual files.