ClickHouse/docs/en/query_language/table_functions/hdfs.md

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')

!!! warning 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')

Original article