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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.

Syntax

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.

RELOAD MODELS​

Note

This statement and SYSTEM RELOAD MODEL merely unload catboost models from the clickhouse-library-bridge. The function catboostEvaluate() loads a model upon first access if it is not loaded yet.

Unloads all CatBoost models.

Syntax

RELOAD MODEL​

Unloads a CatBoost model at model_path.

Syntax

RELOAD FUNCTIONS​

Reloads all registered executable user defined functions or one of them from a configuration file.

Syntax

RELOAD ASYNCHRONOUS METRICS​

Re-calculates all asynchronous metrics. Since asynchronous metrics are periodically updated based on setting asynchronous_metrics_update_period_s, updating them manually using this statement is typically not necessary.

DROP DNS CACHE​

Clears 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_max_entries, dns_cache_update_period parameters.

DROP MARK CACHE​

Clears the mark cache.

DROP REPLICA​

Dead replicas of ReplicatedMergeTree tables can be dropped using following syntax:

Queries will remove the ReplicatedMergeTree 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 DATABASE REPLICA​

Dead replicas of Replicated databases can be dropped using following syntax:

Similar to SYSTEM DROP REPLICA, but removes the Replicated database replica path from ZooKeeper when there's no database to run DROP DATABASE. Please note that it does not remove ReplicatedMergeTree replicas (so you may need SYSTEM DROP REPLICA as well). Shard and replica names are the names that were specified in Replicated engine arguments when creating the database. Also, these names can be obtained from database_shard_name and database_replica_name columns in system.clusters. If the FROM SHARD clause is missing, then replica_name must be a full replica name in shard_name|replica_name format.

DROP UNCOMPRESSED CACHE​

Clears the uncompressed data cache. The uncompressed data cache is enabled/disabled with the query/user/profile-level setting use_uncompressed_cache. Its size can be configured using the server-level setting uncompressed_cache_size.

DROP COMPILED EXPRESSION CACHE​

Clears the compiled expression cache. The compiled expression cache is enabled/disabled with the query/user/profile-level setting compile_expressions.

DROP QUERY CACHE​

Clears the query cache. If a tag is specified, only query cache entries with the specified tag are deleted.

DROP FORMAT SCHEMA CACHE​

Clears cache for schemas loaded from format_schema_path.

Supported formats:

  • Protobuf

FLUSH LOGS​

Flushes buffered log messages to system tables, e.g. system.query_log. Mainly useful for debugging since most system tables have a default flush interval of 7.5 seconds. This will also create system tables even if message queue is empty.

RELOAD CONFIG​

Reloads ClickHouse configuration. Used when configuration is stored in ZooKeeper. Note that SYSTEM RELOAD CONFIG does not reload USER configuration stored in ZooKeeper, it only reloads USER configuration that is stored in users.xml. To reload all USER config use SYSTEM RELOAD USERS

RELOAD USERS​

Reloads all access storages, including: users.xml, local disk access storage, replicated (in ZooKeeper) access storage.

SHUTDOWN​

Not supported in ClickHouse Cloud

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 distributed_foreground_insert setting.

STOP DISTRIBUTED SENDS​

Disables background data distribution when inserting data into distributed tables.

Note

In case of prefer_localhost_replica is enabled (the default), the data to local shard will be inserted anyway.

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.

You can also override some settings via SETTINGS clause, this can be useful to avoid some temporary limitations, like max_concurrent_queries_for_all_users or max_memory_usage.

Note

Each pending block is stored in disk with settings from the initial INSERT query, so that is why sometimes you may want to override settings.

START DISTRIBUTED SENDS​

Enables background data distribution when inserting data into distributed tables.

STOP LISTEN​

Closes the socket and gracefully terminates the existing connections to the server on the specified port with the specified protocol.

However, if the corresponding protocol settings were not specified in the clickhouse-server configuration, this command will have no effect.

  • If CUSTOM 'protocol' modifier is specified, the custom protocol with the specified name defined in the protocols section of the server configuration will be stopped.
  • If QUERIES ALL [EXCEPT .. [,..]] modifier is specified, all protocols are stopped, unless specified with EXCEPT clause.
  • If QUERIES DEFAULT [EXCEPT .. [,..]] modifier is specified, all default protocols are stopped, unless specified with EXCEPT clause.
  • If QUERIES CUSTOM [EXCEPT .. [,..]] modifier is specified, all custom protocols are stopped, unless specified with EXCEPT clause.

START LISTEN​

Allows new connections to be established on the specified protocols.

However, if the server on the specified port and protocol was not stopped using the SYSTEM STOP LISTEN command, this command will have no effect.

Managing MergeTree Tables​

ClickHouse can manage background processes in MergeTree tables.

STOP MERGES​

Not supported in ClickHouse Cloud

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

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​

Not supported in ClickHouse Cloud

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

STOP TTL MERGES​

Provides possibility to stop background delete old data according to TTL expression for tables in the MergeTree family: Returns Ok. even if table does not exist or table has not MergeTree engine. Returns error when database does not exist:

START TTL MERGES​

Provides possibility to start background delete old data according to TTL expression for tables in the MergeTree family: Returns Ok. even if table does not exist. Returns error when database does not exist:

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: Returns Ok. even if table does not exist. Returns error when database does not exist:

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: Returns Ok. even if table does not exist. Returns error when database does not exist:

SYSTEM UNFREEZE​

Clears freezed backup with the specified name from all the disks. See more about unfreezing separate parts in ALTER TABLE table_name UNFREEZE WITH NAME

WAIT LOADING PARTS​

Wait until all asynchronously loading data parts of a table (outdated data parts) will became loaded.

Managing ReplicatedMergeTree Tables​

ClickHouse can manage background replication related processes in ReplicatedMergeTree tables.

STOP FETCHES​

Not supported in ClickHouse Cloud

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 if table or database does not exist.

START FETCHES​

Not supported in ClickHouse Cloud

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 if table or database does not exist.

STOP REPLICATED SENDS​

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

START REPLICATED SENDS​

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

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:

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:

STOP PULLING REPLICATION LOG​

Stops loading new entries from replication log to replication queue in a ReplicatedMergeTree table.

START PULLING REPLICATION LOG​

Cancels SYSTEM STOP PULLING REPLICATION LOG.

SYNC REPLICA​

Wait until a ReplicatedMergeTree table will be synced with other replicas in a cluster, but no more than receive_timeout seconds.

After running this statement the [db.]replicated_merge_tree_family_table_name fetches commands from the common replicated log into its own replication queue, and then the query waits till the replica processes all of the fetched commands. The following modifiers are supported:

  • If a STRICT modifier was specified then the query waits for the replication queue to become empty. The STRICT version may never succeed if new entries constantly appear in the replication queue.
  • If a LIGHTWEIGHT modifier was specified then the query waits only for GET_PART, ATTACH_PART, DROP_RANGE, REPLACE_RANGE and DROP_PART entries to be processed. Additionally, the LIGHTWEIGHT modifier supports an optional FROM 'srcReplicas' clause, where 'srcReplicas' is a comma-separated list of source replica names. This extension allows for more targeted synchronization by focusing only on replication tasks originating from the specified source replicas.
  • If a PULL modifier was specified then the query pulls new replication queue entries from ZooKeeper, but does not wait for anything to be processed.

SYNC DATABASE REPLICA​

Waits until the specified replicated database applies all schema changes from the DDL queue of that database.

Syntax

RESTART REPLICA​

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

RESTORE REPLICA​

Restores a replica if data is [possibly] present but Zookeeper metadata is lost.

Works only on readonly ReplicatedMergeTree tables.

One may execute query after:

  • ZooKeeper root / loss.
  • Replicas path /replicas loss.
  • Individual replica path /replicas/replica_name/ loss.

Replica attaches locally found parts and sends info about them to Zookeeper. Parts present on a replica before metadata loss are not re-fetched from other ones if not being outdated (so replica restoration does not mean re-downloading all data over the network).

Note

Parts in all states are moved to detached/ folder. Parts active before data loss (committed) are attached.

Syntax

Alternative syntax:

Example

Creating a table on multiple servers. After the replica's metadata in ZooKeeper is lost, the table will attach as read-only as metadata is missing. The last query needs to execute on every replica.

Another way:

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

DROP FILESYSTEM CACHE​

Allows to drop filesystem cache.

SYNC FILE CACHE​

Note

It's too heavy and has potential for misuse.

Will do sync syscall.

LOAD PRIMARY KEY​

Load the primary keys for the given table or for all tables.

UNLOAD PRIMARY KEY​

Unload the primary keys for the given table or for all tables.

Managing Refreshable Materialized Views​

Commands to control background tasks performed by Refreshable Materialized Views

Keep an eye on system.view_refreshes while using them.

REFRESH VIEW​

Trigger an immediate out-of-schedule refresh of a given view.

REFRESH VIEW​

Wait for the currently running refresh to complete. If the refresh fails, throws an exception. If no refresh is running, completes immediately, throwing an exception if previous refresh failed.

STOP VIEW, STOP VIEWS​

Disable periodic refreshing of the given view or all refreshable views. If a refresh is in progress, cancel it too.

START VIEW, START VIEWS​

Enable periodic refreshing for the given view or all refreshable views. No immediate refresh is triggered.

CANCEL VIEW​

If there's a refresh in progress for the given view, interrupt and cancel it. Otherwise do nothing.

SYSTEM WAIT VIEW​

Waits for the running refresh to complete. If no refresh is running, returns immediately. If the latest refresh attempt failed, reports an error.

Can be used right after creating a new refreshable materialized view (without EMPTY keyword) to wait for the initial refresh to complete.