4.8 KiB
slug | sidebar_position | sidebar_label |
---|---|---|
/en/sql-reference/table-functions/hdfs | 80 | hdfs |
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}
whereN
,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
Paths may use globbing. Files must match the whole path pattern, not only the suffix or prefix.
*
— Represents arbitrarily many characters except/
but including the empty string.**
— Represents all files inside a folder recursively.?
— Represents an arbitrary single character.{some_string,another_string,yet_another_one}
— Substitutes any of strings'some_string', 'another_string', 'yet_another_one'
. The strings can contain the/
symbol.{N..M}
— Represents any number>= N
and<= M
.
Constructions with {}
are similar to the remote and file table functions.
Example
- 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’
- 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')
- 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. Type:LowCardinalty(String)
._file
— Name of the file. Type:LowCardinalty(String)
._size
— Size of the file in bytes. Type:Nullable(UInt64)
. If the size is unknown, the value isNULL
._time
— Last modified time of the file. Type:Nullable(DateTime)
. If the time is unknown, the value isNULL
.
Hive-style partitioning
When setting use_hive_partitioning
is set to 1, ClickHouse will detect Hive-style partitioning in the path (/name=value/
) and will allow to use partition columns as virtual columns in the query. These virtual columns will have the same names as in the partitioned path, but starting with _
.
Example
Use virtual column, created with Hive-style partitioning
SET use_hive_partitioning = 1;
SELECT * from HDFS('hdfs://hdfs1:9000/data/path/date=*/country=*/code=*/*.parquet') where _date > '2020-01-01' and _country = 'Netherlands' and _code = 42;
Storage Settings
- hdfs_truncate_on_insert - allows to truncate file before insert into it. Disabled by default.
- hdfs_create_new_file_on_insert - 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.
- ignore_access_denied_multidirectory_globs - allows to ignore permission denied errors for multi-directory globs.
See Also