ClickHouse/docs/en/sql-reference/table-functions/file.md

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file

Provides a table-like interface to SELECT from and INSERT to files. This table function is similar to the s3 table function. Use file() when working with local files, and s3() when working with buckets in S3, GCS, or MinIO.

The file function can be used in SELECT and INSERT queries to read from or write to files.

Syntax

file(path [,format] [,structure] [,compression])

Parameters

• path — The relative path to the file from user_files_path. Path to file support following globs in read-only mode: *, ?, {abc,def} and {N..M} where N, M — numbers, 'abc', 'def' — strings.
• format — The format of the file.
• structure — Structure of the table. Format: 'column1_name column1_type, column2_name column2_type, ...'.
• compression — The existing compression type when used in a SELECT query, or the desired compression type when used in an INSERT query. The supported compression types are gz, br, xz, zst, lz4, and bz2.

Returned value

A table with the specified structure for reading or writing data in the specified file.

File Write Examples

Write to a TSV file

INSERT INTO TABLE FUNCTION
file('test.tsv', 'TSV', 'column1 UInt32, column2 UInt32, column3 UInt32')
VALUES (1, 2, 3), (3, 2, 1), (1, 3, 2)

As a result, the data is written into the file test.tsv:

# cat /var/lib/clickhouse/user_files/test.tsv
1	2	3
3	2	1
1	3	2

Partitioned Write to multiple TSV files

If you specify PARTITION BY expression when inserting data into a file() function, a separate file is created for each partition value. Splitting the data into separate files helps to improve reading operations efficiency.

INSERT INTO TABLE FUNCTION
file('test_{_partition_id}.tsv', 'TSV', 'column1 UInt32, column2 UInt32, column3 UInt32')
PARTITION BY column3
VALUES (1, 2, 3), (3, 2, 1), (1, 3, 2)

As a result, the data is written into three files: test_1.tsv, test_2.tsv, and test_3.tsv.

# cat /var/lib/clickhouse/user_files/test_1.tsv
3	2	1

# cat /var/lib/clickhouse/user_files/test_2.tsv
1	3	2

# cat /var/lib/clickhouse/user_files/test_3.tsv
1	2	3

File Read Examples

SELECT from a CSV file

Setting user_files_path and the contents of the file test.csv:

$ grep user_files_path /etc/clickhouse-server/config.xml
    <user_files_path>/var/lib/clickhouse/user_files/</user_files_path>

$ cat /var/lib/clickhouse/user_files/test.csv
    1,2,3
    3,2,1
    78,43,45

Getting data from a table in test.csv and selecting the first two rows from it:

SELECT * FROM
file('test.csv', 'CSV', 'column1 UInt32, column2 UInt32, column3 UInt32')
LIMIT 2;

┌─column1─┬─column2─┬─column3─┐
│       1 │       2 │       3 │
│       3 │       2 │       1 │
└─────────┴─────────┴─────────┘

Getting the first 10 lines of a table that contains 3 columns of UInt32 type from a CSV file:

SELECT * FROM
file('test.csv', 'CSV', 'column1 UInt32, column2 UInt32, column3 UInt32')
LIMIT 10;

Inserting data from a file into a table:

INSERT INTO FUNCTION
file('test.csv', 'CSV', 'column1 UInt32, column2 UInt32, column3 UInt32')
VALUES (1, 2, 3), (3, 2, 1);

SELECT * FROM
file('test.csv', 'CSV', 'column1 UInt32, column2 UInt32, column3 UInt32');

┌─column1─┬─column2─┬─column3─┐
│       1 │       2 │       3 │
│       3 │       2 │       1 │
└─────────┴─────────┴─────────┘

Globs in Path

Multiple path components can have globs. For being processed file must exist and match to the whole path pattern (not only suffix or prefix).

• * — Substitutes any number of any characters except / including empty string.
• ? — Substitutes any single character.
• {some_string,another_string,yet_another_one} — Substitutes any of strings 'some_string', 'another_string', 'yet_another_one', including /.
• {N..M} — Substitutes any number in range from N to M including both borders.
• ** - Fetches all files inside the folder recursively.

Constructions with {} are similar to the remote table function.

Example

Suppose we have several files with the following relative paths:

• 'some_dir/some_file_1'
• 'some_dir/some_file_2'
• 'some_dir/some_file_3'
• 'another_dir/some_file_1'
• 'another_dir/some_file_2'
• 'another_dir/some_file_3'

Query the number of rows in these files:

SELECT count(*) FROM file('{some,another}_dir/some_file_{1..3}', 'TSV', 'name String, value UInt32');

Query the number of rows in all files of these two directories:

SELECT count(*) FROM file('{some,another}_dir/*', 'TSV', 'name String, value UInt32');

:::note If your listing of files contains number ranges with leading zeros, use the construction with braces for each digit separately or use ?. :::

Example

Query the data from files named file000, file001, … , file999:

SELECT count(*) FROM file('big_dir/file{0..9}{0..9}{0..9}', 'CSV', 'name String, value UInt32');

Example

Query the data from all files inside big_dir directory recursively:

SELECT count(*) FROM file('big_dir/**', 'CSV', 'name String, value UInt32');

Example

Query the data from all file002 files from any folder inside big_dir directory recursively:

SELECT count(*) FROM file('big_dir/**/file002', 'CSV', 'name String, value UInt32');

Virtual Columns

• _path — Path to the file.
• _file — Name of the file.

Settings

• engine_file_empty_if_not_exists - allows to select empty data from a file that doesn't exist. Disabled by default.
• engine_file_truncate_on_insert - allows to truncate file before insert into it. Disabled by default.
• engine_file_allow_create_multiple_files - allows to create a new file on each insert if format has suffix. Disabled by default.
• engine_file_skip_empty_files - allows to skip empty files while reading. Disabled by default.
• storage_file_read_method - method of reading data from storage file, one of: read, pread, mmap (only for clickhouse-local). Default value: pread for clickhouse-server, mmap for clickhouse-local.

See Also

• Virtual columns
• Rename files after processing