> ## Documentation Index
> Fetch the complete documentation index at: https://clickhouse.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Polaris catalog

> In this guide, we will walk you through the steps to query your data using ClickHouse and the Snowflake Polaris catalog.

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ClickHouse supports integration with multiple catalogs (Unity, Glue, Polaris,
etc.). In this guide, we will walk you through the steps to query your data
using ClickHouse and the [Apache Polaris Catalog](https://polaris.apache.org/releases/1.1.0/getting-started/using-polaris/#setup).
Apache Polaris supports Iceberg tables and Delta Tables (via Generic Tables). This integration only supports Iceberg tables at this time.

<Note>
  As this feature is experimental, you will need to enable it using:
  `SET allow_experimental_database_unity_catalog = 1;`
</Note>

<h2 id="prerequisites">
  Prerequisites
</h2>

To connect to the Polaris catalog, you will need:

* Snowflake Open Catalog (hosted Polaris) or self-hosted Polaris Catalog
* Your Polaris catalog URI (for example, `https://<account-id>.<region>.aws.snowflakecomputing.com/polaris/api/catalog/v1` or `http://polaris:8181/api/catalog/v1/oauth/tokens`)
* Catalog credentials (client ID and client secret)
* The OAuth tokens URI for your Polaris instance
* Storage endpoint for the object store where your Iceberg data lives (for example, S3)
* ClickHouse version 26.1+

For Open Catalog, Snowflake's managed Polaris offering, your URI will include `/polaris` while for self-hosted, it may not.

<Steps>
  <Step title="Creating a connection between Polaris and ClickHouse" id="connecting">
    Create a database that connects ClickHouse to your Polaris catalog:

    ```sql theme={null}
    CREATE DATABASE polaris_catalog
    ENGINE = DataLakeCatalog('https://<catalog_uri>/api/catalog/v1')
    SETTINGS
        catalog_type = 'rest',
        catalog_credential = '<client-id>:<client-secret>',
        warehouse = 'snowflake',
        auth_scope = 'PRINCIPAL_ROLE:ALL',
        oauth_server_uri = 'https://<catalog_uri>/api/catalog/v1/oauth/tokens',
        storage_endpoint = '<storage_endpoint>'
    ```
  </Step>

  <Step title="Query the Polaris catalog using ClickHouse" id="query-polaris-catalog">
    Once the connection is in place, you can query Polaris:

    ```sql title="Query" theme={null}
    USE polaris_catalog;
    SHOW TABLES;
    ```

    To query a table:

    ```sql title="Query" theme={null}
    SELECT count(*) FROM `polaris_db.my_iceberg_table`;
    ```

    <Note>
      Backticks are required, for example, `schema.table`.
    </Note>

    To inspect the table DDL:

    ```sql theme={null}
    SHOW CREATE TABLE `polaris_db.my_iceberg_table`;
    ```
  </Step>

  <Step title="Loading data from Polaris into ClickHouse" id="loading-data-into-clickhouse">
    To load data from Polaris into a ClickHouse table, create the target table with your desired schema, then insert from the Polaris table:

    ```sql title="Query" theme={null}
    CREATE TABLE my_clickhouse_table
    (
        -- define columns to match your Iceberg table
        `id` Int64,
        `name` String,
        `event_time` DateTime64(3)
    )
    ENGINE = MergeTree
    ORDER BY id;

    INSERT INTO my_clickhouse_table
    SELECT * FROM polaris_catalog.`polaris_db.my_iceberg_table`;
    ```
  </Step>
</Steps>