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

98 lines
3.2 KiB
Markdown
Raw Normal View History

2020-03-20 10:10:48 +00:00
# hdfs {#hdfs}
2019-09-20 11:26:00 +00:00
Creates a table from files in HDFS. This table function is similar to [url](url.md) and [file](file.md) ones.
2020-03-20 10:10:48 +00:00
``` sql
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](../../interfaces/formats.md#formats) 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:
2020-03-20 10:10:48 +00:00
``` sql
SELECT *
FROM hdfs('hdfs://hdfs1:9000/test', 'TSV', 'column1 UInt32, column2 UInt32, column3 UInt32')
LIMIT 2
```
2020-03-20 10:10:48 +00:00
``` text
┌─column1─┬─column2─┬─column3─┐
│ 1 │ 2 │ 3 │
│ 3 │ 2 │ 1 │
└─────────┴─────────┴─────────┘
```
2019-09-04 11:11:30 +00:00
**Globs in path**
2019-09-20 11:26:00 +00:00
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.
2019-09-20 11:26:00 +00:00
2020-03-20 10:10:48 +00:00
Constructions with `{}` are similar to the [remote table function](../../query_language/table_functions/remote.md)).
2019-09-20 11:26:00 +00:00
**Example**
2020-03-20 10:10:48 +00:00
1. Suppose that we have several files with following URIs on HDFS:
2019-09-20 11:26:00 +00:00
- 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
2019-09-20 11:26:00 +00:00
2020-03-20 10:10:48 +00:00
1. Query the amount of rows in these files:
2019-09-20 11:26:00 +00:00
2020-03-20 10:10:48 +00:00
<!-- -->
``` sql
2019-09-20 11:26:00 +00:00
SELECT count(*)
FROM hdfs('hdfs://hdfs1:9000/{some,another}_dir/some_file_{1..3}', 'TSV', 'name String, value UInt32')
```
2020-03-20 10:10:48 +00:00
1. Query the amount of rows in all files of these two directories:
<!-- -->
2019-09-20 11:26:00 +00:00
2020-03-20 10:10:48 +00:00
``` sql
2019-09-20 11:26:00 +00:00
SELECT count(*)
FROM hdfs('hdfs://hdfs1:9000/{some,another}_dir/*', 'TSV', 'name String, value UInt32')
```
2019-09-04 11:11:30 +00:00
2020-03-20 10:10:48 +00:00
!!! warning "Warning"
2019-09-04 11:11:30 +00:00
If your listing of files contains number ranges with leading zeros, use the construction with braces for each digit separately or use `?`.
2019-09-20 11:26:00 +00:00
**Example**
2020-03-20 10:10:48 +00:00
Query the data from files named `file000`, `file001`, … , `file999`:
2019-09-20 11:26:00 +00:00
2020-03-20 10:10:48 +00:00
``` sql
2019-09-20 11:26:00 +00:00
SELECT count(*)
FROM hdfs('hdfs://hdfs1:9000/big_dir/file{0..9}{0..9}{0..9}', 'CSV', 'name String, value UInt32')
```
2019-09-04 11:11:30 +00:00
2020-03-20 10:10:48 +00:00
## Virtual Columns {#virtual-columns}
- `_path` — Path to the file.
- `_file` — Name of the file.
**See Also**
- [Virtual columns](https://clickhouse.tech/docs/en/operations/table_engines/#table_engines-virtual_columns)
2020-01-30 10:34:55 +00:00
[Original article](https://clickhouse.tech/docs/en/query_language/table_functions/hdfs/) <!--hide-->