ClickHouse/docs/en/operations/table_engines/file.md

File(InputFormat)

The data source is a file that stores data in one of the supported input formats (TabSeparated, Native, etc.).

Usage examples:

• Data export from ClickHouse to file.
• Convert data from one format to another.
• Updating data in ClickHouse via editing a file on a disk.

Usage in ClickHouse Server

File(Format)

Format should be supported for either INSERT and SELECT. For the full list of supported formats see Formats.

ClickHouse does not allow to specify filesystem path forFile. It will use folder defined by path setting in server configuration.

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.

You may manually create this subfolder and file in server filesystem and then ATTACH it to table information with matching name, so you can query data from that file.

!!! warning Be careful with this funcionality, 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.

Example:

1. Set up the file_engine_table table:

CREATE TABLE file_engine_table (name String, value UInt32) ENGINE=File(TabSeparated)

By default ClickHouse will create folder /var/lib/clickhouse/data/default/file_engine_table.

2. Manually create /var/lib/clickhouse/data/default/file_engine_table/data.TabSeparated containing:

$ cat data.TabSeparated
one	1
two	2

3. Query the data:

SELECT * FROM file_engine_table

┌─name─┬─value─┐
│ one  │     1 │
│ two  │     2 │
└──────┴───────┘

Usage in Clickhouse-local

In clickhouse-local 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. Example:

$ 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"

Details of Implementation

• Reads can be parallel, but not writes
• Not supported:
  • ALTER
  • SELECT ... SAMPLE
  • Indices
  • Replication

Original article