ClickHouse MergeTree: Overview of ClickHouse Storage Engines

Introduction

ClickHouse supports several storage engines, each optimized for different use cases. Understanding the characteristics of each engine can help you choose the right one for your specific needs, thereby improving performance.

ClickHouse MergeTree Engine Family

Here’s a breakdown of the main storage engines in ClickHouse and their practical use cases:

1. MergeTree Family

• Description: The most versatile and commonly used engine in ClickHouse. It’s designed for high-performance read and write operations and supports data partitioning, primary keys, and data replication.
• Use Cases: Ideal for real-time analytics, time-series data, and any scenario requiring fast writes and reads. It’s highly efficient for large datasets due to its capabilities for data part merging and background optimizations.

2. ReplacingMergeTree

• Description: A variation of the MergeTree engine. It allows for deduplication of rows during merges based on the sorting key.
• Use Cases: Useful in scenarios where you need to remove outdated data or rows are frequently updated. Commonly used in user tracking, where only the latest state of a user session is required.

3. SummingMergeTree

• Description: Another MergeTree variant that sums up numeric columns for rows with the same sorting key during merges.
• Use Cases: Ideal for aggregating metrics or counts over time. Often used in analytical applications for summarizing data.

4. AggregatingMergeTree

• Description: This engine aggregates data during merges using specified aggregate functions.
• Use Cases: Best for cases where pre-aggregated data is necessary, like in business intelligence applications or when dealing with large volumes of aggregated data.

5. CollapsingMergeTree

• Description: A MergeTree variant that collapses rows with the same sorting key during merges, based on a special Sign column.
• Use Cases: Useful for maintaining a versioned dataset where each update is recorded as a new row. Common in scenarios with frequent updates or deletions.

6. VersionedCollapsingMergeTree

• Description: Extends CollapsingMergeTree with version control for rows.
• Use Cases: Ideal for datasets where rows are frequently updated and you need to keep track of changes over time.

7. Log Engine Family (TinyLog, StripeLog, Log)

• Description: Simple engines designed for storing small amounts of data. They lack the sophisticated features of MergeTree engines but offer simplicity and speed for small datasets.
• Use Cases: Best for temporary data, small reference tables, or situations where simplicity and minimal overhead are more important than advanced features.

8. Memory

• Description: Stores data in memory. Data is lost when the server restarts.
• Use Cases: Suitable for caching, temporary data, or testing purposes where persistence is not required.

9. Distributed

• Description: A virtual engine that facilitates distributed query processing across multiple ClickHouse nodes.
• Use Cases: Used for scaling out and parallelizing queries across a cluster of ClickHouse servers. It’s not a standalone storage engine but a way to distribute data stored in other engines.

Conclusion

Each storage engine in ClickHouse is designed for specific types of workloads and data patterns. Choosing the right engine depends on factors like data size, update frequency, query patterns, and the need for real-time analytics. Understanding these nuances allows you to leverage ClickHouse’s full potential by aligning your data storage strategy with your performance and scalability requirements.

To read more about ClickHouse MergeTree, do consider reading the following articles 

• ClickHouse MergeTree: Understanding Use Cases for ClickHouse Storage Engines
• ClickHouse MergeTree: Introduction to ClickHouse Storage Engine Types
• ClickHouse MergeTree: Understanding the Log-Structure MergeTree Algorithm
• ClickHouse MergeTree: Introduction to ReplacingMergeTree
• ClickHouse MergeTree: Introduction to ReplicatedMergeTree