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84 lines
2.9 KiB
Markdown
84 lines
2.9 KiB
Markdown
# File {#table_engines-file}
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The File table engine keeps the data in a file in one of the supported [file
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formats](../../interfaces/formats.md#formats) (TabSeparated, Native, etc.).
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Usage examples:
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- Data export from ClickHouse to file.
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- Convert data from one format to another.
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- Updating data in ClickHouse via editing a file on a disk.
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## Usage in ClickHouse Server
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```sql
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File(Format)
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```
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The `Format` parameter specifies one of the available file formats. To perform
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`SELECT` queries, the format must be supported for input, and to perform
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`INSERT` queries -- for output. The available formats are listed in the
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[Formats](../../interfaces/formats.md#formats) section.
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ClickHouse does not allow to specify filesystem path for`File`. It will use folder defined by [path](../server_settings/settings.md) setting in server configuration.
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When creating table using `File(Format)` it creates empty subdirectory in that folder. When data is written to that table, it's put into `data.Format` file in that subdirectory.
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You may manually create this subfolder and file in server filesystem and then [ATTACH](../../query_language/misc.md) it to table information with matching name, so you can query data from that file.
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!!! warning
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Be careful with this functionality, because ClickHouse does not keep track of external changes to such files. The result of simultaneous writes via ClickHouse and outside of ClickHouse is undefined.
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**Example:**
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**1.** Set up the `file_engine_table` table:
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```sql
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CREATE TABLE file_engine_table (name String, value UInt32) ENGINE=File(TabSeparated)
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```
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By default ClickHouse will create folder `/var/lib/clickhouse/data/default/file_engine_table`.
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**2.** Manually create `/var/lib/clickhouse/data/default/file_engine_table/data.TabSeparated` containing:
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```bash
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$ cat data.TabSeparated
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one 1
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two 2
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```
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**3.** Query the data:
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```sql
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SELECT * FROM file_engine_table
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```
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```text
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┌─name─┬─value─┐
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│ one │ 1 │
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│ two │ 2 │
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└──────┴───────┘
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```
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## Usage in Clickhouse-local
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In [clickhouse-local](../utils/clickhouse-local.md) File engine accepts file path in addition to `Format`. Default input/output streams can be specified using numeric or human-readable names like `0` or `stdin`, `1` or `stdout`.
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**Example:**
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```bash
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$ echo -e "1,2\n3,4" | clickhouse-local -q "CREATE TABLE table (a Int64, b Int64) ENGINE = File(CSV, stdin); SELECT a, b FROM table; DROP TABLE table"
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```
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## Details of Implementation
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- Multiple `SELECT` queries can be performed concurrently, but `INSERT` queries will wait each other.
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- Supported creating new file by `INSERT` query.
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- If file exists, `INSERT` would append new values in it.
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- Not supported:
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- `ALTER`
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- `SELECT ... SAMPLE`
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- Indices
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- Replication
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[Original article](https://clickhouse.yandex/docs/en/operations/table_engines/file/) <!--hide-->
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