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Parts, Partitions, Merges, and Indexes

TL;DR

ClickHouse stores data in immutable parts that are continuously merged in the background. Monitor part counts (100+ per partition degrades performance). Use partitions to organize data and enable fast data management, preferring time-based partitioning over complex multi-column keys. Sparse indexes help locate data quickly, but only for columns in your sorting key. Design your ORDER BY to match your query patterns.

Columnar Storage

ClickHouse is a columnar database: data is stored by columns rather than rows. When you query SELECT user_id, timestamp FROM events, ClickHouse only reads those two columns, not entire rows.

Parts

Data in ClickHouse is stored in parts - immutable chunks written to disk during inserts.

Key characteristics:

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Parts, Partitions, Merges, and Indexes | ClickHouse for Developers