mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-11-29 02:52:13 +00:00
55 lines
3.3 KiB
Markdown
55 lines
3.3 KiB
Markdown
---
|
|
toc_priority: 32
|
|
toc_title: Atomic
|
|
---
|
|
|
|
# Atomic {#atomic}
|
|
|
|
It supports non-blocking [DROP TABLE](#drop-detach-table) and [RENAME TABLE](#rename-table) queries and atomic [EXCHANGE TABLES t1 AND t2](#exchange-tables) queries. `Atomic` database engine is used by default.
|
|
|
|
## Creating a Database {#creating-a-database}
|
|
|
|
``` sql
|
|
CREATE DATABASE test[ ENGINE = Atomic];
|
|
```
|
|
|
|
## Specifics and recommendations {#specifics-and-recommendations}
|
|
|
|
### Table UUID {#table-uuid}
|
|
|
|
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.
|
|
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:
|
|
|
|
```sql
|
|
CREATE TABLE name UUID '28f1c61c-2970-457a-bffe-454156ddcfef' (n UInt64) ENGINE = ...;
|
|
```
|
|
### RENAME TABLE {#rename-table}
|
|
|
|
`RENAME` queries are performed without changing UUID and moving table data. These queries do not wait for the completion of queries using the table and will be executed instantly.
|
|
|
|
### DROP/DETACH TABLE {#drop-detach-table}
|
|
|
|
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 specify by [database_atomic_delay_before_drop_table_sec](../../operations/server-configuration-parameters/settings.md#database_atomic_delay_before_drop_table_sec) setting.
|
|
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.
|
|
|
|
### EXCHANGE TABLES {#exchange-tables}
|
|
|
|
`EXCHANGE` query swaps tables atomically. So instead of this non-atomic operation:
|
|
|
|
```sql
|
|
RENAME TABLE new_table TO tmp, old_table TO new_table, tmp TO old_table;
|
|
```
|
|
you can use one atomic query:
|
|
|
|
``` sql
|
|
EXCHANGE TABLES new_table AND old_table;
|
|
```
|
|
|
|
### ReplicatedMergeTree in Atomic Database {#replicatedmergetree-in-atomic-database}
|
|
|
|
For [ReplicatedMergeTree](../table-engines/mergetree-family/replication.md#table_engines-replication) tables, it is recommended to not specify engine parameters - path in ZooKeeper and replica name. In this case, configuration parameters will be used [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). 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.
|
|
|
|
## See Also
|
|
|
|
- [system.databases](../../operations/system-tables/databases.md) system table
|