# Querying external data in Managed ClickHouse®


Discover federated queries and their capabilities in Managed ClickHouse® and how they simplify and speed up migrating into Exoscale from external data sources.

Federated queries allow communication between Managed ClickHouse and
S3-compatible object storages and web resources. The federated queries
feature in Managed ClickHouse enables you to read and pull data from
an external object storage that uses the S3 integration engine, or any
web resource accessible over HTTP.

> [!NOTE]
> The federated queries feature in Managed ClickHouse is enabled by
> default.

## Why use federated queries

There are a few reasons why you might want to use federated queries:

-   Query remote data from your ClickHouse service. Ingest it into Managed
    ClickHouse® or only reference external data sources as part of an
    analytics query. In the context of an increasing footprint of
    connected data sources, federated queries can help you better
    understand how your customers use your products.
-   Simplify and speed up the import of your data into the Managed ClickHouse instance from a legacy data source, avoiding a long and
    sometimes complex migration path.
-   Improve the migration of data in Managed ClickHouse, and extend
    analysis over external data sources with a relatively low effort in
    comparison to enabling distributed tables and [the remote and
    remoteSecure
    functionalities](https://clickhouse.com/docs/reference/functions/table-functions/remote).

> [!NOTE]
> The `remote()` and `remoteSecure()` features are designed to read from
> remote data sources or provide the ability to create a distributed table
> across remote data sources but they are not designed to read from an
> external S3 storage.

## How it works

To run a federated query, the ClickHouse service user connecting to the
cluster requires grants to the S3 and/or URL sources. The main service
user is granted access to the sources by default, and new users can be
allowed to use the sources via the CREATE TEMPORARY TABLE grant, which
is required for both sources.

For more information on how to enable new users to use the sources,
see [Prerequisites](/product/dbaas/service-specific/clickhouse/how-to/run-federated-queries/#prerequisites).

Federated queries read from external S3-compatible object storage
utilizing the ClickHouse S3 engine. Once you read from a remote
S3-compatible storage, you can select from that storage and insert into
a table in the Exoscale local instance, enabling migration of data into
Exoscale.

For more details on how to run federated queries in Managed ClickHouse,
see
[Run federated queries in Managed ClickHouse®](/product/dbaas/service-specific/clickhouse/how-to/run-federated-queries/).

## Supported sources

Managed ClickHouse queries S3-compatible object storage, Azure Blob Storage, HTTP resources
and external databases such as PostgreSQL and MySQL. You can use them through table
functions, or create persistent tables with the matching table engine (`S3`, `URL`,
`AzureBlobStorage`, `PostgreSQL`, `MySQL`).

Reading from an external source requires the privilege of that source, for example
`GRANT S3` or `GRANT URL`, in addition to the `CREATE TEMPORARY TABLE` grant.

## Related pages

-   [Run federated queries in Managed ClickHouse®](/product/dbaas/service-specific/clickhouse/how-to/run-federated-queries/)
-   [Table functions supported in Managed ClickHouse®](/product/dbaas/service-specific/clickhouse/reference/supported-table-functions/)
-   [Integrating S3 | ClickHouse Docs](https://clickhouse.com/docs/integrations/connectors/data-ingestion/AWS/integrating-s3-with-clickhouse)
-   [remote, remoteSecure | ClickHouse Docs](https://clickhouse.com/docs/reference/functions/table-functions/remote)