The ClickHouse connector allows querying on an external ClickHouse database. This can be used to join data between different systems like ClickHouse and Hive, or between two different ClickHouse instances.
To configure the ClickHouse connector, create a catalog properties file in
etc/catalog named, for example,
clickhouse.properties, to mount the ClickHouse connector as the
clickhouse catalog. Create the file with the following
contents, replacing the connection properties as appropriate for your setup.
Base property setting:
connector.name=clickhouse connection-url=jdbc:clickhouse://example.net:8123 connection-user=username connection-password=yourpassword
- Allow ClickHouse connector to drop table or not
- Enable the query push down feature or not
The push down feature of ClickHouse connector is turn on by default, and you can also set as below:
- Table name is case sensitive or not.
The syntax of ClickHouse is case sensitive. If there are uppercase fields in your database table, you can set them as follows.
Multiple ClickHouse Servers
You can have as many catalogs as you need, so if you have additional ClickHouse servers, simply add another properties file to
etc/catalog with a different name (making sure it ends in
.properties). For example, if
you name the property file
clickhouse2.properties, openLooKeng will create a catalog named
clickhouse2 using the configured connector.
Querying ClickHouse through openLooKeng
The ClickHouse connector provides a schema for every ClickHouse database. You can see the available ClickHouse databases by running
SHOW SCHEMAS FROM clickhouse;
If you have a ClickHouse database named
data, you can view the tables in this database by running
SHOW TABLES FROM clickhouse.data;
You can see a list of the columns in the
hello table in the
data database using either of the following:
DESCRIBE clickhouse.data.hello; SHOW COLUMNS FROM clickhouse.data.hello;
Finally, you can also access the
hello table in the
SELECT * FROM clickhouse.data.hello;
If you used a different name for your catalog properties file, use that catalog name instead of
clickhouse in the above examples.
Mapping Data Types Between openLooKeng and ClickHouse
ClickHouse-to-openLooKeng Type Mapping
openLooKeng support selecting the following ClickHouse Detabase types. The table shows the mapping from ClickHouse data type.
Data type projection table:
|ClickHouse type||openLooKeng type|
openLooKeng-to-ClickHouse Type Mapping
openLooKeng support creating tables with the following type into a ClickHouse Database. The table shows the mapping from openLooKeng to ClickHouse data types.
|openLooKeng type||ClickHouse type|
|TIME WITH TIME ZONE||NA|
|TIMESTAMP WITH TIME ZONE||NA|
Functions that support pushdown
Note: The "$n" is placeholder to present an argument in a function.
count($1) min($1) max($1) sum($1) avg($1) CORR($1,$2) STDDEV($1) stddev_pop($1) stddev_samp($1) skewness($1) kurtosis($1) VARIANCE($1) var_samp($1)
ABS($1) ACOS($1) ASIN($1) ATAN($1) ATAN2($1,$2) CEIL($1) CEILING($1) COS($1) e() EXP($1) FLOOR($1) LN($1) LOG10($1) LOG2($1) MOD($1,$2) pi() POW($1,$2) POWER($1,$2) RAND() RANDOM() ROUND($1) ROUND($1,$2) SIGN($1) SIN($1) SQRT($1) TAN($1)
Functions for Working with Strings
CONCAT($1,$2) LENGTH($1) LOWER($1) LTRIM($1) REPLACE($1,$2) REPLACE($1,$2,$3) RTRIM($1) STRPOS($1,$2) SUBSTR($1,$2,$3) POSITION($1,$2) TRIM($1) UPPER($1)
Functions for Working with Dates and Times
YEAR($1) MONTH($1) QUARTER($1) WEEK($1) DAY($1) HOUR($1) MINUTE($1) SECOND($1) DAY_OF_WEEK($1) DAY_OF_MONTH($1) DAY_OF_YEAR($1)
Note: The functions supported by openLooKeng can also be used in the ClickHouse connector, but functions not in the above list will not be pushed down.
ClickHouse Connector Limitations
CREATE TABLE statement is not supported.
The INSERT statement needs to use CAST, for example, the data type in the table_name_test table is smallint:
insert into table_name_test values (cast(1 as small int));
The ClickHouse syntax supports the use of aliases in where clauses, but not in openLooKeng.
Types such as uuid in ClickHouse are not supported, and all supported types are listed in the mapping table.