ClickHouse connector

The ClickHouse connector allows querying tables in an external ClickHouse server. This can be used to query data in the databases on that server, or combine it with other data from different catalogs accessing ClickHouse or any other supported data source.

Requirements

To connect to a ClickHouse server, you need:

• ClickHouse (version 25.3 or higher) or Altinity (version 22.8 or higher).
• Network access from the Trino coordinator and workers to the ClickHouse server. Port 8123 is the default port.

Configuration

The connector can query a ClickHouse server. Create a catalog properties file that specifies the ClickHouse connector by setting the connector.name to clickhouse.

For example, create the file etc/catalog/example.properties. Replace the connection properties as appropriate for your setup:

connector.name=clickhouse
connection-url=jdbc:clickhouse://host1:8123/
connection-user=exampleuser
connection-password=examplepassword

The connection-url defines the connection information and parameters to pass to the ClickHouse JDBC driver. The supported parameters for the URL are available in the ClickHouse JDBC driver configuration.

The connection-user and connection-password are typically required and determine the user credentials for the connection, often a service user. You can use {doc}secrets </security/secrets> to avoid actual values in the catalog properties files.

(clickhouse-tls)=

Connection security

If you have TLS configured with a globally-trusted certificate installed on your data source, you can enable TLS between your cluster and the data source by appending a parameter to the JDBC connection string set in the connection-url catalog configuration property.

For example, with version 2.6.4 of the ClickHouse JDBC driver, enable TLS by appending the ssl=true parameter to the connection-url configuration property:

connection-url=jdbc:clickhouse://host1:8443/?ssl=true

For more information on TLS configuration options, see the Clickhouse JDBC driver documentation

Multiple ClickHouse servers

If you have multiple ClickHouse servers you need to configure one catalog for each server. To add another catalog:

• Add another properties file to etc/catalog
• Save it with a different name that ends in .properties

For example, if you name the property file sales.properties, Trino uses the configured connector to create a catalog named sales.

Querying ClickHouse

The ClickHouse connector provides a schema for every ClickHouse database. Run SHOW SCHEMAS to see the available ClickHouse databases:

SHOW SCHEMAS FROM example;

If you have a ClickHouse database named web, run SHOW TABLES to view the tables in this database:

SHOW TABLES FROM example.web;

Run DESCRIBE or SHOW COLUMNS to list the columns in the clicks table in the web databases:

DESCRIBE example.web.clicks;
SHOW COLUMNS FROM example.web.clicks;

Run SELECT to access the clicks table in the web database:

SELECT * FROM example.web.clicks;

:::{note} If you used a different name for your catalog properties file, use that catalog name instead of example in the above examples. :::

Table properties

Table property usage example:

CREATE TABLE default.trino_ck (
  id int NOT NULL,
  birthday DATE NOT NULL,
  name VARCHAR,
  age BIGINT,
  logdate DATE NOT NULL
)
WITH (
  engine = 'MergeTree',
  order_by = ARRAY['id', 'birthday'],
  partition_by = ARRAY['toYYYYMM(logdate)'],
  primary_key = ARRAY['id'],
  sample_by = 'id'
);

The following are supported ClickHouse table properties from https://clickhouse.tech/docs/en/engines/table-engines/mergetree-family/mergetree/

Currently the connector only supports Log and MergeTree table engines in create table statement. ReplicatedMergeTree engine is not yet supported.

(clickhouse-type-mapping)=

Type mapping

Because Trino and ClickHouse each support types that the other does not, this connector {ref}modifies some types <type-mapping-overview> when reading or writing data. Data types may not map the same way in both directions between Trino and the data source. Refer to the following sections for type mapping in each direction.

ClickHouse type to Trino type mapping

The connector maps ClickHouse types to the corresponding Trino types according to the following table:

:::{list-table} ClickHouse type to Trino type mapping :widths: 30, 25, 50 :header-rows: 1

• • ClickHouse type
  • Trino type
  • Notes
• • Bool
  • BOOLEAN
• • Int8
  • TINYINT
  • TINYINT and INT1 are aliases of Int8
• • Int16
  • SMALLINT
  • SMALLINT and INT2 are aliases of Int16
• • Int32
  • INTEGER
  • INT, INT4, and INTEGER are aliases of Int32
• • Int64
  • BIGINT
  • BIGINT is an alias of Int64
• • UInt8
  • SMALLINT
• • UInt16
  • INTEGER
• • UInt32
  • BIGINT
• • UInt64
  • DECIMAL(20,0)
• • Float32
  • REAL
  • FLOAT is an alias of Float32
• • Float64
  • DOUBLE
  • DOUBLE is an alias of Float64
• • Decimal
  • DECIMAL
• • FixedString
  • VARBINARY
  • Enabling clickhouse.map-string-as-varchar config property changes the mapping to VARCHAR
• • String
  • VARBINARY
  • Enabling clickhouse.map-string-as-varchar config property changes the mapping to VARCHAR
• • Date
  • DATE
• • DateTime[(timezone)]
  • TIMESTAMP(0) [WITH TIME ZONE]
• • IPv4
  • IPADDRESS
• • IPv6
  • IPADDRESS
• • Enum8
  • VARCHAR
• • Enum16
  • VARCHAR
• • UUID
  • UUID

:::

No other types are supported.

Trino type to ClickHouse type mapping

The connector maps Trino types to the corresponding ClickHouse types according to the following table:

:::{list-table} Trino type to ClickHouse type mapping :widths: 30, 25, 50 :header-rows: 1

• • Trino type
  • ClickHouse type
  • Notes
• • BOOLEAN
  • Bool
• • TINYINT
  • Int8
  • TINYINT and INT1 are aliases of Int8
• • SMALLINT
  • Int16
  • SMALLINT and INT2 are aliases of Int16
• • INTEGER
  • Int32
  • INT, INT4, and INTEGER are aliases of Int32
• • BIGINT
  • Int64
  • BIGINT is an alias of Int64
• • REAL
  • Float32
  • FLOAT is an alias of Float32
• • DOUBLE
  • Float64
  • DOUBLE is an alias of Float64
• • DECIMAL(p,s)
  • Decimal(p,s)
• • VARCHAR
  • String
• • CHAR
  • String
• • VARBINARY
  • String
  • Enabling clickhouse.map-string-as-varchar config property changes the mapping to VARCHAR
• • DATE
  • Date
• • TIMESTAMP(0)
  • DateTime
• • UUID
  • UUID

:::

No other types are supported.

(clickhouse-sql-support)=

SQL support

The connector provides read and write access to data and metadata in a ClickHouse catalog. In addition to the globally available and read operation statements, the connector supports the following features:

• , see also
• , see also:

(clickhouse-insert)=

(clickhouse-alter-table)=

(clickhouse-procedures)=

Procedures

(clickhouse-table-functions)=

Table functions

The connector provides specific {doc}table functions </functions/table> to access ClickHouse.

(clickhouse-query-function)=

query(varchar) -> table

The query function allows you to query the underlying database directly. It requires syntax native to ClickHouse, because the full query is pushed down and processed in ClickHouse. This can be useful for accessing native features which are not available in Trino or for improving query performance in situations where running a query natively may be faster.

As a simple example, query the example catalog and select an entire table:

SELECT
  *
FROM
  TABLE(
    example.system.query(
      query => 'SELECT
        *
      FROM
        tpch.nation'
    )
  );

Performance

The connector includes a number of performance improvements, detailed in the following sections.

(clickhouse-pushdown)=

Pushdown

The connector supports pushdown for a number of operations:

• {ref}limit-pushdown

{ref}Aggregate pushdown <aggregation-pushdown> for the following functions:

• {func}avg
• {func}count
• {func}max
• {func}min
• {func}sum