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104 lines
3.6 KiB
Markdown
104 lines
3.6 KiB
Markdown
# HDFS {#table_engines-hdfs}
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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.htmll)via ClickHouse. This engine is similar
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to the [File](file.md) and [URL](url.md) engines, but provides Hadoop-specific features.
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## Usage
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```sql
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ENGINE = HDFS(URI, format)
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```
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The `URI` parameter is the whole file URI in HDFS.
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The `format` parameter specifies one of the available file formats. To perform
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`SELECT` queries, the format must be supported for input, and to perform
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`INSERT` queries -- for output. The available formats are listed in the
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[Formats](../../interfaces/formats.md#formats) section.
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The path part of `URI` may contain globs. In this case the table would be readonly.
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**Example:**
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**1.** Set up the `hdfs_engine_table` table:
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```sql
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CREATE TABLE hdfs_engine_table (name String, value UInt32) ENGINE=HDFS('hdfs://hdfs1:9000/other_storage', 'TSV')
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```
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**2.** Fill file:
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```sql
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INSERT INTO hdfs_engine_table VALUES ('one', 1), ('two', 2), ('three', 3)
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```
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**3.** Query the data:
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```sql
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SELECT * FROM hdfs_engine_table LIMIT 2
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```
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```text
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┌─name─┬─value─┐
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│ one │ 1 │
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│ two │ 2 │
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└──────┴───────┘
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```
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## Implementation Details
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- Reads and writes can be parallel
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- Not supported:
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- `ALTER` and `SELECT...SAMPLE` operations.
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- Indexes.
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- Replication.
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**Globs in path**
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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).
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- `*` — Substitutes any number of any characters except `/` including empty string.
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- `?` — Substitutes any single character.
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- `{some_string,another_string,yet_another_one}` — Substitutes any of strings `'some_string', 'another_string', 'yet_another_one'`.
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- `{N..M}` — Substitutes any number in range from N to M including both borders.
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Constructions with `{}` are similar to the [remote table function](../../query_language/table_functions/remote.md)).
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**Example**
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1. Suppose we have several files in TSV format with the following URIs on HDFS:
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- 'hdfs://hdfs1:9000/some_dir/some_file_1'
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- 'hdfs://hdfs1:9000/some_dir/some_file_2'
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- 'hdfs://hdfs1:9000/some_dir/some_file_3'
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- 'hdfs://hdfs1:9000/another_dir/some_file_1'
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- 'hdfs://hdfs1:9000/another_dir/some_file_2'
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- 'hdfs://hdfs1:9000/another_dir/some_file_3'
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2. There are several ways to make a table consisting of all six files:
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```sql
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CREATE TABLE table_with_range (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/{some,another}_dir/some_file_{1..3}', 'TSV')
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```
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Another way:
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```sql
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CREATE TABLE table_with_question_mark (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/{some,another}_dir/some_file_?', 'TSV')
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```
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Table consists of all the files in both directories (all files should satisfy format and schema described in query):
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```sql
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CREATE TABLE table_with_asterisk (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/{some,another}_dir/*', 'TSV')
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```
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!!! warning
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If the listing of files contains number ranges with leading zeros, use the construction with braces for each digit separately or use `?`.
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**Example**
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Create table with files named `file000`, `file001`, ... , `file999`:
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```sql
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CREARE TABLE big_table (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/big_dir/file{0..9}{0..9}{0..9}', 'CSV')
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```
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[Original article](https://clickhouse.yandex/docs/en/operations/table_engines/hdfs/) <!--hide-->
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