ClickHouse connector#

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

Configuration#

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

For example, to access an instance as myclickhouse, create the file etc/catalog/myclickhouse.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

Multiple ClickHouse servers#

If you have multiple ClickHouse instances you need to configure one catalog for each instance. 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 myclickhouse;

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

SHOW TABLES FROM myclickhouse.web;

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

DESCRIBE myclickhouse.web.clicks;
SHOW COLUMNS FROM clickhouse.web.clicks;

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

SELECT * FROM myclickhouse.web.clicks;

Note

If you used a different name for your catalog properties file, use that catalog name instead of myclickhouse in the above examples.

Table properties#

Table property usage example:

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

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

Property Name	Default Value	Description
`engine`	`Log`	Name and parameters of the engine.
`order_by`	(none)	list of columns to be the sorting key. Required if `engine` is `MergeTree`.
`partition_by`	(none)	list of columns to be the partition key. Optional.
`primary_key`	(none)	list of columns to be the primary key. Optional.
`sample_by`	(none)	An expression for sampling. Optional.

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

Pushdown#

The connector supports pushdown for a number of operations:

Limit pushdown

Aggregate pushdown for the following functions:

Limitations#

The following SQL statements aren’t supported: