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The explanation about `clusterAllReplicas` is not clear. This change tries to simplify the sentence and clarify the explanation.
60 lines
2.7 KiB
Markdown
60 lines
2.7 KiB
Markdown
---
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slug: /en/sql-reference/table-functions/cluster
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sidebar_position: 30
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sidebar_label: cluster
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title: "cluster, clusterAllReplicas"
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---
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Allows to access all shards (configured in the `remote_servers` section) of a cluster without creating a [Distributed](../../engines/table-engines/special/distributed.md) table. Only one replica of each shard is queried.
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`clusterAllReplicas` function — same as `cluster`, but all replicas are queried. Each replica in a cluster is used as a separate shard/connection.
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:::note
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All available clusters are listed in the [system.clusters](../../operations/system-tables/clusters.md) table.
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:::
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**Syntax**
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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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**Arguments**
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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, set `default` if not specified.
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- `db.table` or `db`, `table` - Name of a database and a table.
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- `sharding_key` - A sharding key. Optional. Needs to be specified if the cluster has more than one shard.
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**Returned value**
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The dataset from clusters.
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**Using Macros**
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`cluster_name` can contain macros — substitution in curly brackets. The substituted value is taken from the [macros](../../operations/server-configuration-parameters/settings.md#macros) section of the server configuration file.
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Example:
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```sql
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SELECT * FROM cluster('{cluster}', default.example_table);
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```
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**Usage and Recommendations**
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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 do not 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#skip_unavailable_shards)
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- [load_balancing](../../operations/settings/settings.md#load_balancing)
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