Allows to access all shards in an existing cluster which configured in
remote_servers section without creating a Distributed 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.
All available clusters are listed in the
cluster('cluster_name', db.table[, sharding_key]) cluster('cluster_name', db, table[, sharding_key]) clusterAllReplicas('cluster_name', db.table[, sharding_key]) clusterAllReplicas('cluster_name', db, table[, sharding_key])
cluster_name – Name of a cluster that is used to build a set of addresses and connection parameters to remote and local servers.
sharding_key - When insert into cluster function with more than one shard, sharding_key need to be provided.
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
clusterAllReplicas table functions.
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
secure are taken from
<remote_servers> config section. See details in Distributed engine.