🛰️ `ClickHouse` Native Protocol Rust Client w/ Arrow Compatibility

ClickHouse access in rust over ClickHouse's native protocol.

Currently supports revision 54477, DBMS_MIN_REVISION_WITH_QUERY_PLAN_SERIALIZATION, the latest revision as of June 2025.

A high-performance, async Rust client for ClickHouse with native Arrow integration. Designed to be faster and more memory-efficient than existing alternatives.

Why clickhouse-arrow?

🚀 Performance: Optimized for speed with zero-copy deserialization where possible
🎯 Arrow Native: First-class Apache Arrow support for efficient data interchange
📊 90%+ Test Coverage: Comprehensive test suite ensuring reliability
🔄 Async/Await: Modern async API built on Tokio
🗜️ Compression: LZ4 and ZSTD support for efficient data transfer
☁️ Cloud Ready: Full ClickHouse Cloud compatibility
🛡️ Type Safe: Compile-time type checking with the #[derive(Row)] macro

Details

The crate supports two "modes" of operation:

`ArrowFormat`

Support allowing interoperability with arrow.

`NativeFormat`

Uses internal types and custom traits if a dependency on arrow is not required.

[!NOTE] I am considering putting arrow behind a feature flag to provide a subset of features without arrow dependencies. If that sounds interesting to you, let me know.

`CreateOptions`, `SchemaConversions`, and Schemas

Creating Tables from Arrow Schemas

clickhouse-arrow provides powerful DDL capabilities through CreateOptions, allowing you to create ClickHouse tables directly from Arrow schemas:

use clickhouse_arrow::{Client, ArrowFormat, CreateOptions};
use arrow::datatypes::{Schema, Field, DataType};

// Define your Arrow schema
let schema = Schema::new(vec![
    Field::new("id", DataType::UInt64, false),
    Field::new("name", DataType::Utf8, false),
    Field::new("status", DataType::Dictionary(Box::new(DataType::Int8), Box::new(DataType::Utf8)), false),
]);

// Configure table creation
let options = CreateOptions::new("MergeTree")
    .with_order_by(&["id".to_string()])
    .with_partition_by("toYYYYMM(created_at)")
    .with_setting("index_granularity", 8192);

// Create the table
client.create_table(None, "my_table", &schema, &options, None).await?;

Schema Conversions for Type Control

SchemaConversions (type alias for HashMap<String, Type>) provides fine-grained control over Arrow-to-ClickHouse type mappings. This is especially important for:

Converting Dictionary → Enum: By default, Arrow Dictionary types map to LowCardinality(String). Use SchemaConversions to map them to Enum8 or Enum16 instead:

use clickhouse_arrow::{Type, CreateOptions};
use std::collections::HashMap;

let schema_conversions = HashMap::from([
    // Convert status column from Dictionary to Enum8
    ("status".to_string(), Type::Enum8(vec![
        ("active".to_string(), 0),
        ("inactive".to_string(), 1),
        ("pending".to_string(), 2),
    ])),
    // Convert category to Enum16 for larger enums
    ("category".to_string(), Type::Enum16(vec![
        ("electronics".to_string(), 0),
        ("clothing".to_string(), 1),
        // ... up to 65k values
    ])),
]);

let options = CreateOptions::new("MergeTree")
    .with_order_by(&["id".to_string()])
    .with_schema_conversions(schema_conversions);

Geo Types: Preserve geographic types during conversion
Date Types: Choose between Date and Date32
Custom Type Mappings: Override any default type conversion

Field Naming Constants

When working with complex Arrow types, use these constants to ensure compatibility:

use clickhouse_arrow::arrow::types::*;

// For List types - inner field is named "item"
let list_field = Field::new("data", DataType::List(
    Arc::new(Field::new(LIST_ITEM_FIELD_NAME, DataType::Int32, true))
), true);

// For Struct/Tuple types - fields are named "field_0", "field_1", etc.
let tuple_fields = vec![
    Field::new(format!("{}{}", TUPLE_FIELD_NAME_PREFIX, 0), DataType::Int32, false),
    Field::new(format!("{}{}", TUPLE_FIELD_NAME_PREFIX, 1), DataType::Utf8, false),
];

// For Map types - uses specific field names
let map_type = DataType::Map(
    Arc::new(Field::new(MAP_FIELD_NAME, DataType::Struct(
        vec![
            Field::new(STRUCT_KEY_FIELD_NAME, DataType::Utf8, false),
            Field::new(STRUCT_VALUE_FIELD_NAME, DataType::Int32, true),
        ].into()
    ), false)),
    false
);

These constants ensure your Arrow schemas align with ClickHouse's expectations and maintain compatibility with arrow-rs conventions.

Performance & Benchmarks

Benchmark Results

The following benchmarks were run on [TODO: Remove - Add your machine specs, e.g., M2 MacBook Pro, 16GB RAM]:

Insert Performance

[TODO: Remove - Add insert benchmark results]

Query Performance

[TODO: Remove - Add query benchmark results]

Memory Usage

[TODO: Remove - Add memory usage comparisons]

Key Performance Features

Compression Benefits: LZ4/ZSTD compression often improves performance over remote networks due to reduced network I/O and can sometimes speed up even on localhost due to deterministic deserialization patterns
Zero-Copy: Arrow integration enables zero-copy data transfer where possible
Streaming: Large datasets are processed in chunks to maintain low memory footprint
Connection Pooling: The pool feature enables connection reuse for better throughput

Running Benchmarks

# Run all benchmarks
cd clickhouse-arrow && cargo bench --features test_utils

# Run specific benchmark
cd clickhouse-arrow && cargo bench --bench insert --features test_utils

# Run with release-lto profile for best performance
cd clickhouse-arrow && cargo bench --profile release-lto --features test_utils

Queries

Query Settings

The clickhouse_arrow::Settings type allows configuring ClickHouse query settings. You can import it directly:

use clickhouse_arrow::Settings;
// or via prelude
use clickhouse_arrow::prelude::*;

Refer to the settings module documentation for details and examples.

Arrow Round-Trip

There are cases where a round trip may deserialize a different type by schema or array than the schema and array you used to create the table.

will try to maintain an accurate and updated list as they occur. In addition, when possible, I will provide options or other functionality to alter this behavior.

`(String|Binary)View`/`Large(List|String|Binary)` variations are normalized.

Behavior: ClickHouse does not make the same distinction between Utf8, Utf8View, or LargeUtf8. All of these are mapped to either Type::Binary (the default, see above) or Type::String
Option: None
Default: Unsupported
Impact: When deserializing from ClickHouse, manual modification will be necessary to use these data types.

`Utf8` -> `Binary`

Behavior: By default, Type::String/DataType::Utf8 will be represented as Binary.
Option: strings_as_strings (default: false).
Default: Disabled (false).
Impact: Set to true to strip map Type::String -> DataType::Utf8. Binary tends to be more efficient to work with in high throughput scenarios

Nullable `Array`s

Behavior: ClickHouse does not allow Nullable(Array(...)), but insertion with non-null data is allowed by default. To modify this behavior, set array_nullable_error to true.
Option: array_nullable_error (default: false).
Default: Disabled (false).
Impact: Enables flexible insertion but may cause schema mismatches if nulls are present.

`LowCardinality(Nullable(...))` vs `Nullable(LowCardinality(...))`

Behavior: Like arrays mentioned above, ClickHouse does not allow nullable low cardinality. The default behavior is to push down the nullability.
Option: low_cardinality_nullable_error (default: false).
Default: Disabled (false).
Impact: Enables flexible insertion but may cause schema mismatches if nulls are present.

`Enum8`/`Enum16` vs. `LowCardinality`

Behavior: Arrow Dictionary types map to LowCardinality, but ClickHouse Enum types may also map to Dictionary, altering the type on round-trip.
Option: No options available rather provide hash maps for either enum_i8 and/or enum_i16 for CreateOptions during schema creation.
Impact: The default behavior will ignore enums when starting from arrow.

[!NOTE] For examples of these cases, refer to the tests in the module arrow::types

[!NOTE] The configuration for the options above can be found in options

[!NOTE] For a builder of create options use during schema creation (eg Engine, Order By, Enum8 and Enum16 lookups), refer to CreateOptions

clickhouse-arrow 0.1.1

🛰️ ClickHouse Native Protocol Rust Client w/ Arrow Compatibility