Indexes

Log-Structured Merge Tree

ClickHouse supports primary key indexes. The indexing mechanism is called a sparse index. In the MergeTree, data are sorted by primary key lexicographically in each part. Then ClickHouse selects some marks for every Nth row, where N is chosen adaptively by default. Together these marks serve as a sparse index, which allows efficient range queries.

Storage Model

Decomposition Storage Model (Columnar)

ClickHouse is a column-oriented DBMS and it stores data by columns.

Query Execution

Vectorized Model

Storage Architecture

Disk-oriented In-Memory Hybrid

ClickHouse has multiple types of table engines. The type of the table engine determines where the data is stored, concurrent level, whether indexes are supported and some other properties. Key table engine family for production use is a MergeTree that allows for resilient storage of large volumes of data and supports replication. There's also a Log family for lightweight storage of temporary data and Distributed engine for querying a cluster.

Foreign Keys

Not Supported

ClickHouse does not support foreign keys.

Query Interface

Custom API SQL HTTP / REST Command-line / Shell

ClickHouses provides two types of parsers: a full SQL parser and a data format parser. It uses SQL parser for all types of queries and the data format parser only for INSERT queries. Beyond the query language, it provides multiple user interfaces, including HTTP interface, JDBC driver, TCP interface, command-line client, etc.

Storage Organization

Indexed Sequential Access Method (ISAM) Sorted Files

Views

Virtual Views Materialized Views

ClickHouse supports both virtual views and materialized views. The materialized views store data transformed by corresponding SELECT query. The SELECT query can contain DISTINCT, GROUP BY, ORDER BY, LIMIT, etc.

Logging

Physical Logging

ClickHouse replicates its data on multiple nodes and monitors data synchronicity on replicas. It recovers after failures by syncing data from other replica nodes.

Query Compilation

Code Generation

ClickHouse supports runtime code generation. The code is generated for every kind of query on the fly, removing all indirection and dynamic dispatch. Runtime code generation can be better when it fuses many operations together and fully utilizes CPU execution units.

Stored Procedures

Not Supported

Currently, stored procedures and UDF are listed as open issues in ClickHouse.

Joins

Hash Join

ClickHouse uses hash join by default, which is done by placing the right part of data in a hash table in memory. If there's not enough memory for hash join it falls back to merge join.

System Architecture

Shared-Nothing

ClickHouse system in a distributed setup is a cluster of shards. It uses asynchronous multimaster replication and there is no single point of contention across the system.

Compression

Dictionary Encoding Delta Encoding Naïve (Page-Level)

In addition to general-purpose encoding with LZ4 (default) or Zstd, ClickHouse supports dictionary encoding via LowCardinality data type, as well as delta, double-delta and Gorilla encodings via column codecs.

Parallel Execution

Intra-Operator (Horizontal) Inter-Operator (Vertical)

ClickHouse utilizes half cores for single-node queries and one replica of each shard for distributed queries by default. It could be tuned to utilize only one core, all cores of the whole cluster or anything in between.

Data Model

Relational

ClickHouse uses the relational database model.

Concurrency Control

Not Supported

ClickHouse does not support multi-statement transactions.