ClickHouse/docs/en/sql-reference/table-functions/cluster.md
2020-10-13 20:23:29 +03:00

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---
toc_priority: 50
toc_title: cluster
---
# cluster, clusterAllReplicas {#cluster-clusterallreplicas}
Allows to access all shards in an existing cluster which configured in `remote_servers` section without creating a [Distributed](../../engines/table-engines/special/distributed.md) table. One replica of each shard is queried.
`clusterAllReplicas` - same as `cluster` but all replicas are queried. Each replica in a cluster is used as separate shard/connection.
!!! note "Note"
All available clusters are listed in the `system.clusters` table.
Signatures:
``` sql
cluster('cluster_name', db.table)
cluster('cluster_name', db, table)
clusterAllReplicas('cluster_name', db.table)
clusterAllReplicas('cluster_name', db, table)
```
`cluster_name` Name of a cluster that is used to build a set of addresses and connection parameters to remote and local servers.
Using the `cluster` and `clusterAllReplicas` table functions are less efficient than creating a `Distributed` table because in this case, the server connection is re-established for every request. When processing a large number of queries, please always create the `Distributed` table ahead of time, and dont use the `cluster` and `clusterAllReplicas` table functions.
The `cluster` and `clusterAllReplicas` table functions can be useful in the following cases:
- Accessing a specific cluster for data comparison, debugging, and testing.
- Queries to various ClickHouse clusters and replicas for research purposes.
- Infrequent distributed requests that are made manually.
Connection settings like `host`, `port`, `user`, `password`, `compression`, `secure` are taken from `<remote_servers>` config section. See details in [Distributed engine](../../engines/table-engines/special/distributed.md).
**See Also**
- [skip_unavailable_shards](../../operations/settings/settings.md#settings-skip_unavailable_shards)
- [load_balancing](../../operations/settings/settings.md#settings-load_balancing)