mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-09-24 18:50:49 +00:00
a2047de471
* [docs] introduction for integration table engines * Update jdbc.md * Update odbc.md * Update mysql.md * Update kafka.md * Update hdfs.md
122 lines
4.1 KiB
Markdown
122 lines
4.1 KiB
Markdown
---
|
||
toc_priority: 4
|
||
toc_title: HDFS
|
||
---
|
||
|
||
# HDFS {#table_engines-hdfs}
|
||
|
||
This engine provides integration with [Apache Hadoop](https://en.wikipedia.org/wiki/Apache_Hadoop) ecosystem by allowing to manage data on [HDFS](https://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-hdfs/HdfsDesign.html)via ClickHouse. This engine is similar
|
||
to the [File](../../../engines/table-engines/special/file.md#table_engines-file) and [URL](../../../engines/table-engines/special/url.md#table_engines-url) engines, but provides Hadoop-specific features.
|
||
|
||
## Usage {#usage}
|
||
|
||
``` sql
|
||
ENGINE = HDFS(URI, format)
|
||
```
|
||
|
||
The `URI` parameter is the whole file URI in HDFS.
|
||
The `format` parameter specifies one of the available file formats. To perform
|
||
`SELECT` queries, the format must be supported for input, and to perform
|
||
`INSERT` queries – for output. The available formats are listed in the
|
||
[Formats](../../../interfaces/formats.md#formats) section.
|
||
The path part of `URI` may contain globs. In this case the table would be readonly.
|
||
|
||
**Example:**
|
||
|
||
**1.** Set up the `hdfs_engine_table` table:
|
||
|
||
``` sql
|
||
CREATE TABLE hdfs_engine_table (name String, value UInt32) ENGINE=HDFS('hdfs://hdfs1:9000/other_storage', 'TSV')
|
||
```
|
||
|
||
**2.** Fill file:
|
||
|
||
``` sql
|
||
INSERT INTO hdfs_engine_table VALUES ('one', 1), ('two', 2), ('three', 3)
|
||
```
|
||
|
||
**3.** Query the data:
|
||
|
||
``` sql
|
||
SELECT * FROM hdfs_engine_table LIMIT 2
|
||
```
|
||
|
||
``` text
|
||
┌─name─┬─value─┐
|
||
│ one │ 1 │
|
||
│ two │ 2 │
|
||
└──────┴───────┘
|
||
```
|
||
|
||
## Implementation Details {#implementation-details}
|
||
|
||
- Reads and writes can be parallel
|
||
- Not supported:
|
||
- `ALTER` and `SELECT...SAMPLE` operations.
|
||
- Indexes.
|
||
- Replication.
|
||
|
||
**Globs in path**
|
||
|
||
Multiple path components can have globs. For being processed file should exists and matches to the whole path pattern. Listing of files determines during `SELECT` (not at `CREATE` moment).
|
||
|
||
- `*` — 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](../../../sql-reference/table-functions/remote.md) table function.
|
||
|
||
**Example**
|
||
|
||
1. Suppose we have several files in TSV format with the 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’
|
||
|
||
1. There are several ways to make a table consisting of all six files:
|
||
|
||
<!-- -->
|
||
|
||
``` sql
|
||
CREATE TABLE table_with_range (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/{some,another}_dir/some_file_{1..3}', 'TSV')
|
||
```
|
||
|
||
Another way:
|
||
|
||
``` sql
|
||
CREATE TABLE table_with_question_mark (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/{some,another}_dir/some_file_?', 'TSV')
|
||
```
|
||
|
||
Table consists of all the files in both directories (all files should satisfy format and schema described in query):
|
||
|
||
``` sql
|
||
CREATE TABLE table_with_asterisk (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/{some,another}_dir/*', 'TSV')
|
||
```
|
||
|
||
!!! warning "Warning"
|
||
If the listing of files contains number ranges with leading zeros, use the construction with braces for each digit separately or use `?`.
|
||
|
||
**Example**
|
||
|
||
Create table with files named `file000`, `file001`, … , `file999`:
|
||
|
||
``` sql
|
||
CREARE TABLE big_table (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/big_dir/file{0..9}{0..9}{0..9}', 'CSV')
|
||
```
|
||
|
||
## Virtual Columns {#virtual-columns}
|
||
|
||
- `_path` — Path to the file.
|
||
- `_file` — Name of the file.
|
||
|
||
**See Also**
|
||
|
||
- [Virtual columns](../../../engines/table-engines/index.md#table_engines-virtual_columns)
|
||
|
||
[Original article](https://clickhouse.tech/docs/en/operations/table_engines/hdfs/) <!--hide-->
|