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41 lines
2.1 KiB
Markdown
41 lines
2.1 KiB
Markdown
---
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toc_priority: 50
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toc_title: cluster
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---
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# cluster, clusterAllReplicas {#cluster-clusterallreplicas}
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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.
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`clusterAllReplicas` - same as `cluster` but all replicas are queried. Each replica in a cluster is used as separate shard/connection.
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!!! note "Note"
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All available clusters are listed in the `system.clusters` table.
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Signatures:
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``` sql
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cluster('cluster_name', db.table[, sharding_key])
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cluster('cluster_name', db, table[, sharding_key])
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clusterAllReplicas('cluster_name', db.table[, sharding_key])
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clusterAllReplicas('cluster_name', db, table[, sharding_key])
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```
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`cluster_name` – Name of a cluster that is used to build a set of addresses and connection parameters to remote and local servers.
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`sharding_key` - When insert into cluster function with more than one shard, sharding_key need to be provided.
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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 don’t use the `cluster` and `clusterAllReplicas` table functions.
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The `cluster` and `clusterAllReplicas` table functions can be useful in the following cases:
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- Accessing a specific cluster for data comparison, debugging, and testing.
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- Queries to various ClickHouse clusters and replicas for research purposes.
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- Infrequent distributed requests that are made manually.
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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).
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**See Also**
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- [skip_unavailable_shards](../../operations/settings/settings.md#settings-skip_unavailable_shards)
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- [load_balancing](../../operations/settings/settings.md#settings-load_balancing)
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