Creating a connection to ClickHouse® from DataLens On-premises

To create a ClickHouse® connection:

1. Go to the workbook page or create a new one.

2. In the top-right corner, click Create → Connection.

3. Select the ClickHouse® connection.

4. Specify the connection parameters for the external ClickHouse® database:

   • Host name: Specify the path to a master host or a ClickHouse® master host IP address. You can specify multiple hosts in a comma-separated list. If you fail to connect to the first host, DataLens will select the next one from the list.

   • HTTP interface port: Specify the ClickHouse® connection port. The default port is 8443.

   • Username: Specify a username for the ClickHouse® connection.

     Warning

     The user must have the readonly parameter set to one of the following values:

     • 0: Allows all queries.

     • 1: Allows only data read queries. In this case, specify the following in the ClickHouse® settings:

       • join_use_nulls = 1
       • send_progress_in_http_headers = 0
       • output_format_json_quote_denormals = 1

       For DataLens, set the Readonly parameter to 1 in the connection's advanced settings.

     • 2: Allows queries to read data and edit settings.

   • Password: Specify a password for the user.

   • Cache TTL in seconds: Specify cache TTL or leave the default value. The recommended value is 300 seconds (5 minutes).

   • Raw SQL level: Enables you to use an ad-hoc SQL query to generate a dataset. This option is disabled by default. When enabling it, you will need to select the raw SQL level:

     • Subqueries only: Describe dataset sources using SQL queries.
     • Subqueries and parameters: Describe dataset sources using SQL queries and use source parameterization.
     • SQL to read: Describe dataset sources using SQL queries, use source parameterization, create QL charts, and execute read-only SQL queries.

5. Optionally, test the connection by clicking Check connection.

6. Click Create connection.

7. Enter a name for the connection and click Create.

Additional settingsAdditional settings

You can specify additional connection settings under Advanced connection settings:

• TLS: If this option is enabled, the database is accessed over HTTPS; if not, over HTTP.

• CA Certificate: To upload a certificate, click Attach file and select the certificate file. When the certificate is uploaded, the field shows the file name.

• Disable data export: When this option is on, the data export item will not be available in the menu for the charts based on this connection. However, you will still be able to copy chart data and take screenshots.

• Readonly: Select the permission for queries to read data, write data, and change parameters. This setting value must not exceed the user's corresponding setting in ClickHouse®:

  • 0: Allows all queries.
  • 1: Allows only data read queries.
  • 2: Allows queries to read data and edit settings.

Specifics of using a connection to ClickHouse®Specifics of using a connection to ClickHouse®

In ClickHouse®, you can create a dataset on top of a VIEW that contains the JOIN section. To do this, make sure a view is created with the join_use_nulls option enabled. We recommend setting join_use_nulls = 1 in the SETTINGS section:

CREATE VIEW ... (
    ...
) AS
    SELECT
        ...
    FROM
        ...
    SETTINGS join_use_nulls = 1

You should also enable this option for raw-sql subqueries that are used as a data source in your dataset.

To avoid errors when using views with the JOIN section in DataLens, re-create all views and set join_use_nulls = 1. This fills in empty cells with NULL values and converts the type of the relevant fields to Nullable.

ClickHouse® is a registered trademark of ClickHouse, Inc.