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

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hdfs

Creates a table from files in HDFS. This table function is similar to url and file ones.

hdfs(URI, format, structure)

Input parameters

• URI — The relative URI to the file in HDFS. Path to file support following globs in readonly 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, ...'.

Returned value

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

Example

Table from hdfs://hdfs1:9000/test and selection of the first two rows from it:

SELECT *
FROM hdfs('hdfs://hdfs1:9000/test', 'TSV', 'column1 UInt32, column2 UInt32, column3 UInt32')
LIMIT 2

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

Globs in path

Multiple path components can have globs. For being processed file should exists and matches 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'.
• {N..M} — Substitutes any number in range from N to M including both borders.

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

Example

1. Suppose that we have several files with following URIs on HDFS:

• ‘hdfs://hdfs1:9000/some_dir/some_file_1’
• ‘hdfs://hdfs1:9000/some_dir/some_file_2’
• ‘hdfs://hdfs1:9000/some_dir/some_file_3’
• ‘hdfs://hdfs1:9000/another_dir/some_file_1’
• ‘hdfs://hdfs1:9000/another_dir/some_file_2’
• ‘hdfs://hdfs1:9000/another_dir/some_file_3’

2. Query the amount of rows in these files:

SELECT count(*)
FROM hdfs('hdfs://hdfs1:9000/{some,another}_dir/some_file_{1..3}', 'TSV', 'name String, value UInt32')

3. Query the amount of rows in all files of these two directories:

SELECT count(*)
FROM hdfs('hdfs://hdfs1:9000/{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 hdfs('hdfs://hdfs1:9000/big_dir/file{0..9}{0..9}{0..9}', 'CSV', 'name String, value UInt32')

Virtual Columns

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

Storage Settings

• hdfs_truncate_on_insert - allows to truncate file before insert into it. Disabled by default.
• hdfs_create_multiple_files - allows to create a new file on each insert if format has suffix. Disabled by default.
• hdfs_skip_empty_files - allows to skip empty files while reading. Disabled by default.

See Also

• Virtual columns