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55 lines
3.4 KiB
Markdown
55 lines
3.4 KiB
Markdown
---
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toc_priority: 32
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toc_title: Atomic
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---
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# Atomic {#atomic}
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It supports non-blocking [DROP TABLE](#drop-detach-table) and [RENAME TABLE](#rename-table) queries and atomic [EXCHANGE TABLES](#exchange-tables) queries. `Atomic` database engine is used by default.
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## Creating a Database {#creating-a-database}
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``` sql
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CREATE DATABASE test [ENGINE = Atomic];
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```
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## Specifics and recommendations {#specifics-and-recommendations}
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### Table UUID {#table-uuid}
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All tables in database `Atomic` have persistent [UUID](../../sql-reference/data-types/uuid.md) and store data in directory `/clickhouse_path/store/xxx/xxxyyyyy-yyyy-yyyy-yyyy-yyyyyyyyyyyy/`, where `xxxyyyyy-yyyy-yyyy-yyyy-yyyyyyyyyyyy` is UUID of the table.
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Usually, the UUID is generated automatically, but the user can also explicitly specify the UUID in the same way when creating the table (this is not recommended). To display the `SHOW CREATE` query with the UUID you can use setting [show_table_uuid_in_table_create_query_if_not_nil](../../operations/settings/settings.md#show_table_uuid_in_table_create_query_if_not_nil). For example:
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```sql
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CREATE TABLE name UUID '28f1c61c-2970-457a-bffe-454156ddcfef' (n UInt64) ENGINE = ...;
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```
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### RENAME TABLE {#rename-table}
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[RENAME](../../sql-reference/statements/rename.md) queries are performed without changing UUID and moving table data. These queries do not wait for the completion of queries using the table and are executed instantly.
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### DROP/DETACH TABLE {#drop-detach-table}
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On `DROP TABLE` no data is removed, database `Atomic` just marks table as dropped by moving metadata to `/clickhouse_path/metadata_dropped/` and notifies background thread. Delay before final table data deletion is specified by the [database_atomic_delay_before_drop_table_sec](../../operations/server-configuration-parameters/settings.md#database_atomic_delay_before_drop_table_sec) setting.
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You can specify synchronous mode using `SYNC` modifier. Use the [database_atomic_wait_for_drop_and_detach_synchronously](../../operations/settings/settings.md#database_atomic_wait_for_drop_and_detach_synchronously) setting to do this. In this case `DROP` waits for running `SELECT`, `INSERT` and other queries which are using the table to finish. Table will be actually removed when it's not in use.
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### EXCHANGE TABLES/DICTIONARIES {#exchange-tables}
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[EXCHANGE](../../sql-reference/statements/exchange.md) query swaps tables or dictionaries atomically. For instance, instead of this non-atomic operation:
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```sql
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RENAME TABLE new_table TO tmp, old_table TO new_table, tmp TO old_table;
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```
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you can use one atomic query:
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``` sql
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EXCHANGE TABLES new_table AND old_table;
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```
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### ReplicatedMergeTree in Atomic Database {#replicatedmergetree-in-atomic-database}
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For [ReplicatedMergeTree](../table-engines/mergetree-family/replication.md#table_engines-replication) tables, it is recommended not to specify engine parameters - path in ZooKeeper and replica name. In this case, configuration parameters [default_replica_path](../../operations/server-configuration-parameters/settings.md#default_replica_path) and [default_replica_name](../../operations/server-configuration-parameters/settings.md#default_replica_name) will be used. If you want to specify engine parameters explicitly, it is recommended to use `{uuid}` macros. This is useful so that unique paths are automatically generated for each table in ZooKeeper.
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## See Also
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- [system.databases](../../operations/system-tables/databases.md) system table
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