ClickHouse/docs/en/query_language/system.md

SYSTEM Queries

• RELOAD DICTIONARIES
• RELOAD DICTIONARY
• DROP DNS CACHE
• DROP MARK CACHE
• FLUSH LOGS
• RELOAD CONFIG
• SHUTDOWN
• KILL
• STOP DISTRIBUTED SENDS
• FLUSH DISTRIBUTED
• START DISTRIBUTED SENDS
• STOP MERGES
• START MERGES

RELOAD DICTIONARIES

Reloads all dictionaries that have been successfully loaded before. By default, dictionaries are loaded lazily (see dictionaries_lazy_load), so instead of being loaded automatically at startup, they are initialized on first access through dictGet function or SELECT from tables with ENGINE = Dictionary. The SYSTEM RELOAD DICTIONARIES query reloads such dictionaries (LOADED). Always returns Ok. regardless of the result of the dictionary update.

RELOAD DICTIONARY dictionary_name

Completely reloads a dictionary dictionary_name, regardless of the state of the dictionary (LOADED / NOT_LOADED / FAILED). Always returns Ok. regardless of the result of updating the dictionary. The status of the dictionary can be checked by querying the system.dictionaries table.

SELECT name, status FROM system.dictionaries;

DROP DNS CACHE

Resets ClickHouse's internal DNS cache. Sometimes (for old ClickHouse versions) it is necessary to use this command when changing the infrastructure (changing the IP address of another ClickHouse server or the server used by dictionaries).

For more convenient (automatic) cache management, see disable_internal_dns_cache, dns_cache_update_period parameters.

DROP MARK CACHE

Resets the mark cache. Used in development of ClickHouse and performance tests.

FLUSH LOGS

Flushes buffers of log messages to system tables (e.g. system.query_log). Allows you to not wait 7.5 seconds when debugging.

RELOAD CONFIG

Reloads ClickHouse configuration. Used when configuration is stored in ZooKeeeper.

SHUTDOWN

Normally shuts down ClickHouse (like service clickhouse-server stop / kill {$pid_clickhouse-server})

KILL

Aborts ClickHouse process (like kill -9 {$ pid_clickhouse-server})

Managing Distributed Tables

ClickHouse can manage distributed tables. When a user inserts data into these tables, ClickHouse first creates a queue of the data that should be sent to cluster nodes, then asynchronously sends it. You can manage queue processing with the STOP DISTRIBUTED SENDS, FLUSH DISTRIBUTED, and START DISTRIBUTED SENDS queries. You can also synchronously insert distributed data with the insert_distributed_sync setting.

STOP DISTRIBUTED SENDS

Disables background data distribution when inserting data into distributed tables.

SYSTEM STOP DISTRIBUTED SENDS [db.]<distributed_table_name>

FLUSH DISTRIBUTED

Forces ClickHouse to send data to cluster nodes synchronously. If any nodes are unavailable, ClickHouse throws an exception and stops query execution. You can retry the query until it succeeds, which will happen when all nodes are back online.

SYSTEM FLUSH DISTRIBUTED [db.]<distributed_table_name>

START DISTRIBUTED SENDS

Enables background data distribution when inserting data into distributed tables.

SYSTEM START DISTRIBUTED SENDS [db.]<distributed_table_name>

STOP MERGES

Provides possibility to stop background merges for tables in the MergeTree family:

SYSTEM STOP MERGES [[db.]merge_tree_family_table_name]

!!! note "Note" DETACH / ATTACH table will start background merges for the table even in case when merges have been stopped for all MergeTree tables before.

START MERGES

Provides possibility to start background merges for tables in the MergeTree family:

SYSTEM START MERGES [[db.]merge_tree_family_table_name]

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