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ClickHouse/docs/en/sql-reference/statements/system.md
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DOCSUP-3871: Document prefer_not_to_merge option (#17090) 
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36 SYSTEM

SYSTEM Statements

The list of available SYSTEM statements:

RELOAD EMBEDDED DICTIONARIES

Reload all Internal dictionaries. By default, internal dictionaries are disabled. Always returns Ok. regardless of the result of the internal dictionary update.

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

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.

DROP REPLICA

Dead replicas can be dropped using following syntax:

SYSTEM DROP REPLICA 'replica_name' FROM TABLE database.table;
SYSTEM DROP REPLICA 'replica_name' FROM DATABASE database;
SYSTEM DROP REPLICA 'replica_name';
SYSTEM DROP REPLICA 'replica_name' FROM ZKPATH '/path/to/table/in/zk';

Queries will remove the replica path in ZooKeeper. It is useful when the replica is dead and its metadata cannot be removed from ZooKeeper by DROP TABLE because there is no such table anymore. It will only drop the inactive/stale replica, and it cannot drop local replica, please use DROP TABLE for that. DROP REPLICA does not drop any tables and does not remove any data or metadata from disk.

The first one removes metadata of 'replica_name' replica of database.table table. The second one does the same for all replicated tables in the database. The third one does the same for all replicated tables on the local server. The fourth one is useful to remove metadata of dead replica when all other replicas of a table were dropped. It requires the table path to be specified explicitly. It must be the same path as was passed to the first argument of ReplicatedMergeTree engine on table creation.

DROP UNCOMPRESSED CACHE

Reset the uncompressed data cache. Used in development of ClickHouse and performance tests. For manage uncompressed data cache parameters use following server level settings uncompressed_cache_size and query/user/profile level settings use_uncompressed_cache

DROP COMPILED EXPRESSION CACHE

Reset the compiled expression cache. Used in development of ClickHouse and performance tests. Complied expression cache used when query/user/profile enable option compile

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. This will also create system tables even if message queue is empty.

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>

Managing MergeTree Tables

ClickHouse can manage background processes in MergeTree tables.

STOP MERGES

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

SYSTEM STOP MERGES [ON VOLUME <volume_name> | [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 [ON VOLUME <volume_name> | [db.]merge_tree_family_table_name]

STOP TTL MERGES

Provides possibility to stop background delete old data according to TTL expression for tables in the MergeTree family: Return Ok. even table doesn’t exists or table have not MergeTree engine. Return error when database doesn’t exists:

SYSTEM STOP TTL MERGES [[db.]merge_tree_family_table_name]

START TTL MERGES

Provides possibility to start background delete old data according to TTL expression for tables in the MergeTree family: Return Ok. even table doesn’t exists. Return error when database doesn’t exists:

SYSTEM START TTL MERGES [[db.]merge_tree_family_table_name]

STOP MOVES

Provides possibility to stop background move data according to TTL table expression with TO VOLUME or TO DISK clause for tables in the MergeTree family: Return Ok. even table doesn’t exists. Return error when database doesn’t exists:

SYSTEM STOP MOVES [[db.]merge_tree_family_table_name]

START MOVES

Provides possibility to start background move data according to TTL table expression with TO VOLUME and TO DISK clause for tables in the MergeTree family: Return Ok. even table doesn’t exists. Return error when database doesn’t exists:

SYSTEM STOP MOVES [[db.]merge_tree_family_table_name]

Managing ReplicatedMergeTree Tables

ClickHouse can manage background replication related processes in ReplicatedMergeTree tables.

STOP FETCHES

Provides possibility to stop background fetches for inserted parts for tables in the ReplicatedMergeTree family: Always returns Ok. regardless of the table engine and even table or database doesn’t exists.

SYSTEM STOP FETCHES [[db.]replicated_merge_tree_family_table_name]

START FETCHES

Provides possibility to start background fetches for inserted parts for tables in the ReplicatedMergeTree family: Always returns Ok. regardless of the table engine and even table or database doesn’t exists.

SYSTEM START FETCHES [[db.]replicated_merge_tree_family_table_name]

STOP REPLICATED SENDS

Provides possibility to stop background sends to other replicas in cluster for new inserted parts for tables in the ReplicatedMergeTree family:

SYSTEM STOP REPLICATED SENDS [[db.]replicated_merge_tree_family_table_name]

START REPLICATED SENDS

Provides possibility to start background sends to other replicas in cluster for new inserted parts for tables in the ReplicatedMergeTree family:

SYSTEM START REPLICATED SENDS [[db.]replicated_merge_tree_family_table_name]

STOP REPLICATION QUEUES

Provides possibility to stop background fetch tasks from replication queues which stored in Zookeeper for tables in the ReplicatedMergeTree family. Possible background tasks types - merges, fetches, mutation, DDL statements with ON CLUSTER clause:

SYSTEM STOP REPLICATION QUEUES [[db.]replicated_merge_tree_family_table_name]

START REPLICATION QUEUES

Provides possibility to start background fetch tasks from replication queues which stored in Zookeeper for tables in the ReplicatedMergeTree family. Possible background tasks types - merges, fetches, mutation, DDL statements with ON CLUSTER clause:

SYSTEM START REPLICATION QUEUES [[db.]replicated_merge_tree_family_table_name]

SYNC REPLICA

Wait until a ReplicatedMergeTree table will be synced with other replicas in a cluster. Will run until receive_timeout if fetches currently disabled for the table.

SYSTEM SYNC REPLICA [db.]replicated_merge_tree_family_table_name

RESTART REPLICA

Provides possibility to reinitialize Zookeeper sessions state for ReplicatedMergeTree table, will compare current state with Zookeeper as source of true and add tasks to Zookeeper queue if needed Initialization replication quene based on ZooKeeper date happens in the same way as ATTACH TABLE statement. For a short time the table will be unavailable for any operations.

SYSTEM RESTART REPLICA [db.]replicated_merge_tree_family_table_name

RESTART REPLICAS

Provides possibility to reinitialize Zookeeper sessions state for all ReplicatedMergeTree tables, will compare current state with Zookeeper as source of true and add tasks to Zookeeper queue if needed

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