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 20.8 or higher.
Network access from the Presto 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, to access a server as clickhouse, create the file etc/catalog/clickhouse.properties. Replace the connection properties as appropriate for your setup:

connector.name=clickhouse
clickhouse.connection-url=jdbc:clickhouse://host1:8123/
clickhouse.connection-user=default
clickhouse.connection-password=secret

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 clickhouse.properties, Prestodb uses the configured connector to create a catalog named clickhouse.

General configuration properties¶

The following table describes general catalog configuration properties for the connector:

Property Name	Default Value	Description
`clickhouse.map-string-as-varchar`	false	When creating a table, support the clickhouse data type String.
`clickhouse.allow-drop-table`	false	Allow delete table operation.

Querying ClickHouse¶

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

SHOW SCHEMAS FROM clickhouse;

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

SHOW TABLES FROM clickhouse.tpch;

Run DESCRIBE or SHOW COLUMNS to list the columns in the cks table in the tpch databases:

DESCRIBE clickhouse.tpch.cks;
SHOW COLUMNS FROM clickhouse.tpch.cks;

Run SELECT to access the cks table in the tpch database:

SELECT * FROM clickhouse.tpch.cks;

Note

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

Table properties¶

Table property usage example:

CREATE TABLE default.prestodb_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/

Property Name	Default Value	Description
`engine`	`Log`	Name and parameters of the engine.
`order_by`	(none)	Array of columns or expressions to concatenate to create the sorting key. Required if `engine` is `MergeTree`.
`partition_by`	(none)	Array of columns or expressions to use as nested partition keys. Optional.
`primary_key`	(none)	Array of columns or expressions to concatenate to create the primary key. Optional.
`sample_by`	(none)	An expression to use 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

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: