Part 3: The MergeTree Family
Understanding the core engine of ClickHouse: MergeTree, Sorting, and Partitioning.
If ClickHouse is a race car, the MergeTree engine is its V12 engine. It’s the default and most versatile table engine in ClickHouse.
What is MergeTree?
The MergeTree family of engines is designed for inserting
very large amounts of data into a table. The data is quickly written
to the table part by part, and then rules are applied for merging these
parts in the background.
Key Concepts
Primary Key
Unlike other DBs, the primary key in ClickHouse is not unique. It’s used for sorting data and index granularity.
Partitioning
Partitioning splits your data into logical blocks (e.g., by month) to optimize queries and data management.
Creating a Table
CREATE TABLE hits ( timestamp DateTime, user_id UInt64, url String, event_type Enum(‘view’ = 1, ‘click’ = 2) ) ENGINE = MergeTree() PARTITION BY toYYYYMM(timestamp) ORDER BY (timestamp, user_id);
How Merges Work
When you insert data, ClickHouse creates a new “part” on disk. In the background, ClickHouse merges these parts together to keep the number of files manageable and data sorted.
Conclusion
Understanding MergeTree is crucial for mastering ClickHouse. In the next part, we’ll look at how to model your data effectively.