ClickHouse connector#

The ClickHouse connector allows querying tables in an external Yandex 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 21.3 or higher) or Altinity (version 20.8 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, 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
connection-url=jdbc:clickhouse://host1:8123/
connection-user=exampleuser
connection-password=examplepassword

Note

Trino uses the new ClickHouse driver(com.clickhouse.jdbc.ClickHouseDriver) by default, but the new driver only supports ClickHouse server with version >= 20.7.

For compatibility with ClickHouse server versions < 20.7, you can temporarily continue to use the old ClickHouse driver(ru.yandex.clickhouse.ClickHouseDriver) by adding the following catalog property: clickhouse.legacy-driver=true.

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://host1:8123/?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.

General configuration properties#

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

Property name	Description	Default value
`case-insensitive-name-matching`	Support case insensitive schema and table names.	`false`
`case-insensitive-name-matching.cache-ttl`		`1m`
`case-insensitive-name-matching.config-file`	Path to a name mapping configuration file in JSON format that allows Trino to disambiguate between schemas and tables with similar names in different cases.	`null`
`case-insensitive-name-matching.refresh-period`	Frequency with which Trino checks the name matching configuration file for changes.	`0` (refresh disabled)
`metadata.cache-ttl`	Duration for which metadata, including table and column statistics, is cached.	`0` (caching disabled)
`metadata.cache-missing`	Cache the fact that metadata, including table and column statistics, is not available	`false`
`metadata.cache-maximum-size`	Maximum number of objects stored in the metadata cache	`10000`
`write.batch-size`	Maximum number of statements in a batched execution. Do not change this setting from the default. Non-default values may negatively impact performance.	`1000`

Procedures#

system.flush_metadata_cache()

Flush JDBC metadata caches. For example, the following system call flushes the metadata caches for all schemas in the example catalog
```
USE example.myschema;
CALL system.flush_metadata_cache();
```

Case insensitive matching#

When case-insensitive-name-matching is set to true, Trino is able to query non-lowercase schemas and tables by maintaining a mapping of the lowercase name to the actual name in the remote system. However, if two schemas and/or tables have names that differ only in case (such as “customers” and “Customers”) then Trino fails to query them due to ambiguity.

In these cases, use the case-insensitive-name-matching.config-file catalog configuration property to specify a configuration file that maps these remote schemas/tables to their respective Trino schemas/tables:

{
  "schemas": [
    {
      "remoteSchema": "CaseSensitiveName",
      "mapping": "case_insensitive_1"
    },
    {
      "remoteSchema": "cASEsENSITIVEnAME",
      "mapping": "case_insensitive_2"
    }],
  "tables": [
    {
      "remoteSchema": "CaseSensitiveName",
      "remoteTable": "tablex",
      "mapping": "table_1"
    },
    {
      "remoteSchema": "CaseSensitiveName",
      "remoteTable": "TABLEX",
      "mapping": "table_2"
    }]
}

Queries against one of the tables or schemes defined in the mapping attributes are run against the corresponding remote entity. For example, a query against tables in the case_insensitive_1 schema is forwarded to the CaseSensitiveName schema and a query against case_insensitive_2 is forwarded to the cASEsENSITIVEnAME schema.

At the table mapping level, a query on case_insensitive_1.table_1 as configured above is forwarded to CaseSensitiveName.tablex, and a query on case_insensitive_1.table_2 is forwarded to CaseSensitiveName.TABLEX.

By default, when a change is made to the mapping configuration file, Trino must be restarted to load the changes. Optionally, you can set the case-insensitive-name-mapping.refresh-period to have Trino refresh the properties without requiring a restart:

case-insensitive-name-mapping.refresh-period=30s

Non-transactional INSERT#

The connector supports adding rows using INSERT statements. By default, data insertion is performed by writing data to a temporary table. You can skip this step to improve performance and write directly to the target table. Set the insert.non-transactional-insert.enabled catalog property or the corresponding non_transactional_insert catalog session property to true.

Note that with this property enabled, data can be corrupted in rare cases where exceptions occur during the insert operation. With transactions disabled, no rollback can be performed.

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 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.

Type mapping#

The data type mappings are as follows:

ClickHouse	Trino	Notes
`Int8`	`TINYINT`	`TINYINT`, `BOOL`, `BOOLEAN` 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`	`TIMESTAMP`
`IPv4`	`IPADDRESS`
`IPv6`	`IPADDRESS`
`Enum8`	`VARCHAR`
`Enum16`	`VARCHAR`
`UUID`	`UUID`

Type mapping configuration properties#

The following properties can be used to configure how data types from the connected data source are mapped to Trino data types and how the metadata is cached in Trino.

Property name

Description

Default value

unsupported-type-handling

Configure how unsupported column data types are handled:

IGNORE, column is not accessible.
CONVERT_TO_VARCHAR, column is converted to unbounded VARCHAR.

The respective catalog session property is unsupported_type_handling.

IGNORE

jdbc-types-mapped-to-varchar

Allow forced mapping of comma separated lists of data types to convert to unbounded VARCHAR

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:

ALTER SCHEMA#

The connector supports renaming a schema with the ALTER SCHEMA RENAME statement. ALTER SCHEMA SET AUTHORIZATION is not supported.

Performance#

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

Pushdown#

The connector supports pushdown for a number of operations:

Limit pushdown

Aggregate pushdown for the following functions:

Predicate pushdown support#

The connector does not support pushdown of any predicates on columns with textual types like CHAR or VARCHAR. This ensures correctness of results since the data source may compare strings case-insensitively.

In the following example, the predicate is not pushed down for either query since name is a column of type VARCHAR:

SELECT * FROM nation WHERE name > 'CANADA';
SELECT * FROM nation WHERE name = 'CANADA';