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

# ClickHouse

> Connect Cube to ClickHouse for low-latency analytics, including host credentials and driver-specific connection options.

[ClickHouse](https://clickhouse.com) is a fast and resource efficient
[open-source database](https://github.com/ClickHouse/ClickHouse) for real-time
applications and analytics.

## Prerequisites

* The hostname for the [ClickHouse][clickhouse] database server
* The [username/password][clickhouse-docs-users] for the
  [ClickHouse][clickhouse] database server

## Setup

### Manual

Add the following to a `.env` file in your Cube project:

```dotenv theme={"dark"}
CUBEJS_DB_TYPE=clickhouse
CUBEJS_DB_HOST=my.clickhouse.host
CUBEJS_DB_NAME=my_clickhouse_database
CUBEJS_DB_USER=clickhouse_user
CUBEJS_DB_PASS=**********
```

## Environment Variables

| Environment Variable                                                                                                  | Description                                                                        | Possible Values           | Required |
| --------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------- | ------------------------- | :------: |
| [`CUBEJS_DB_HOST`](/reference/configuration/environment-variables#cubejs_db_host)                                     | The host URL for a database                                                        | A valid database host URL |     ✅    |
| [`CUBEJS_DB_PORT`](/reference/configuration/environment-variables#cubejs_db_port)                                     | The port for the database connection                                               | A valid port number       |     ❌    |
| [`CUBEJS_DB_NAME`](/reference/configuration/environment-variables#cubejs_db_name)                                     | The name of the database to connect to                                             | A valid database name     |     ✅    |
| [`CUBEJS_DB_USER`](/reference/configuration/environment-variables#cubejs_db_user)                                     | The username used to connect to the database                                       | A valid database username |     ✅    |
| [`CUBEJS_DB_PASS`](/reference/configuration/environment-variables#cubejs_db_pass)                                     | The password used to connect to the database                                       | A valid database password |     ✅    |
| [`CUBEJS_DB_CLICKHOUSE_READONLY`](/reference/configuration/environment-variables#cubejs_db_clickhouse_readonly)       | Whether the ClickHouse user has read-only access or not                            | `true`, `false`           |     ❌    |
| [`CUBEJS_DB_CLICKHOUSE_COMPRESSION`](/reference/configuration/environment-variables#cubejs_db_clickhouse_compression) | Whether the ClickHouse client has compression enabled or not                       | `true`, `false`           |     ❌    |
| [`CUBEJS_DB_MAX_POOL`](/reference/configuration/environment-variables#cubejs_db_max_pool)                             | The maximum number of concurrent database connections to pool. Default is `20`     | A valid number            |     ❌    |
| [`CUBEJS_CONCURRENCY`](/reference/configuration/environment-variables#cubejs_concurrency)                             | The number of [concurrent queries][ref-data-source-concurrency] to the data source | A valid number            |     ❌    |

[ref-data-source-concurrency]: /admin/connect-to-data/concurrency#data-source-concurrency

## Pre-Aggregation Feature Support

When using [pre-aggregations][ref-preaggs] with ClickHouse, you have to define
[indexes][ref-preaggs-indexes] in pre-aggregations. Otherwise, you might get
the following error: `ClickHouse doesn't support pre-aggregations without indexes`.

### `count_distinct_approx`

Measures of type
[`count_distinct_approx`][ref-schema-ref-types-formats-countdistinctapprox] can
not be used in pre-aggregations when using ClickHouse as a source database.

### `rollup_join`

You can use [`rollup_join` pre-aggregations][ref-preaggs-rollup-join] to join
data from ClickHouse and other data sources inside Cube Store.

Alternatively, you can leverage ClickHouse support for [integration table
engines](https://clickhouse.com/docs/en/engines/table-engines#integration-engines)
to join data from ClickHouse and other data sources inside ClickHouse.
To do so, define table engines in ClickHouse and connect your ClickHouse as the
only data source to Cube.

## Pre-Aggregation Build Strategies

<Info>
  To learn more about pre-aggregation build strategies, [head
  here][ref-caching-using-preaggs-build-strats].
</Info>

| Feature       | Works with read-only mode? | Is default? |
| ------------- | :------------------------: | :---------: |
| Batching      |              ✅             |      ✅      |
| Export Bucket |              ✅             |      -      |

By default, ClickHouse uses [batching][self-preaggs-batching] to build
pre-aggregations.

### Batching

No extra configuration is required to configure batching for ClickHouse.

### Export Bucket

<Warning>
  Clickhouse driver **only** supports using AWS S3 for export buckets.
</Warning>

#### AWS S3

For [improved pre-aggregation performance with large
datasets][ref-caching-large-preaggs], enable export bucket functionality by
configuring Cube with the following environment variables:

<Info>
  Ensure the AWS credentials are correctly configured in IAM to allow reads and
  writes to the export bucket in S3.
</Info>

```dotenv theme={"dark"}
CUBEJS_DB_EXPORT_BUCKET_TYPE=s3
CUBEJS_DB_EXPORT_BUCKET=my.bucket.on.s3
CUBEJS_DB_EXPORT_BUCKET_AWS_KEY=<AWS_KEY>
CUBEJS_DB_EXPORT_BUCKET_AWS_SECRET=<AWS_SECRET>
CUBEJS_DB_EXPORT_BUCKET_AWS_REGION=<AWS_REGION>
```

## SSL

To enable SSL-encrypted connections between Cube and ClickHouse, set the
[`CUBEJS_DB_SSL`](/reference/configuration/environment-variables#cubejs_db_ssl) environment variable to `true`. For more information on how to
configure custom certificates, please check out [Enable SSL Connections to the
Database][ref-recipe-enable-ssl].

## Additional Configuration

You can connect to a ClickHouse database when your user's permissions are
[restricted][clickhouse-readonly] to read-only, by setting
[`CUBEJS_DB_CLICKHOUSE_READONLY`](/reference/configuration/environment-variables#cubejs_db_clickhouse_readonly) to `true`.

You can connect to a ClickHouse database with compression enabled, by setting
[`CUBEJS_DB_CLICKHOUSE_COMPRESSION`](/reference/configuration/environment-variables#cubejs_db_clickhouse_compression) to `true`.

[clickhouse]: https://clickhouse.tech/

[clickhouse-docs-users]: https://clickhouse.tech/docs/en/operations/settings/settings-users/

[clickhouse-readonly]: https://clickhouse.com/docs/en/operations/settings/permissions-for-queries#readonly

[ref-caching-using-preaggs-build-strats]: /docs/pre-aggregations/using-pre-aggregations#pre-aggregation-build-strategies

[ref-recipe-enable-ssl]: /recipes/configuration/using-ssl-connections-to-data-source

[ref-caching-large-preaggs]: /docs/pre-aggregations/using-pre-aggregations#export-bucket

[ref-schema-ref-types-formats-countdistinctapprox]: /reference/data-modeling/measures#type

[self-preaggs-batching]: #batching

[ref-preaggs]: /docs/pre-aggregations/using-pre-aggregations

[ref-preaggs-indexes]: /reference/data-modeling/pre-aggregations#indexes

[ref-preaggs-rollup-join]: /reference/data-modeling/pre-aggregations#rollup_join