SYSTEM Queries

Managing Distributed Tables

ClickHouse can manage distributed tables. When a user inserts data into such table, ClickHouse creates a queue of the data which should be sent to servers of the cluster, then asynchronously sends them. You can control the processing of queue by using the requests STOP DISTRIBUTED SENDS, FLUSH DISTRIBUTED and START DISTRIBUTED SENDS. Also, you can synchronously insert distributed data with the insert_distributed_sync setting.

STOP DISTRIBUTED SENDS

Disables background data distributing, when inserting data into the distributed tables.

SYSTEM STOP DISTRIBUTED SENDS [db.]<distributed_table_name>

FLUSH DISTRIBUTED

Forces ClickHouse to send data to the servers of the cluster in synchronous mode. If some of the servers are not available, ClickHouse throws an exception and stops query processing. When servers are back into operation, you should repeat the query.

SYSTEM FLUSH DISTRIBUTED [db.]<distributed_table_name>

START DISTRIBUTED SENDS

Enables background data distributing, when inserting data into the distributed tables.

SYSTEM START DISTRIBUTED SENDS [db.]<distributed_table_name>