ClickHouse/docs/en/operations/table_engines/hdfs.md

# 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](file.md) and [URL](url.md) engines, but provides Hadoop-specific features.

## 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

- 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](../../query_language/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'

2. 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
    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

- `_path` — Path to the file.
- `_file` — Name of the file.

**See Also**

- [Virtual columns](https://clickhouse.yandex/docs/en/operations/table_engines/#table_engines-virtual_columns)

[Original article](https://clickhouse.yandex/docs/en/operations/table_engines/hdfs/) <!--hide-->