Clickhouse之HDFS(Hadoop分布式文件存储系统)

This engine provides integration with the Apache Hadoop ecosystem by allowing to manage data on HDFS via ClickHouse. This engine is similar to the File and URL engines, but provides Hadoop-specific features.

Usage

ENGINE = HDFS(URI, format)

Engine Parameters

• URI - whole file URI in HDFS. The path part of URI may contain globs. In this case the table would be readonly.
• format - 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 section.

Example:

1. Set up the hdfs_engine_table table:

CREATE TABLE hdfs_engine_table (name String, value UInt32) ENGINE=HDFS('hdfs://hdfs1:9000/other_storage', 'TSV')

2. Fill file:

INSERT INTO hdfs_engine_table VALUES ('one', 1), ('two', 2), ('three', 3)

3. Query the data:

SELECT * FROM hdfs_engine_table LIMIT 2

┌─name─┬─value─┐
│ one  │     1 │
│ two  │     2 │
└──────┴───────┘

Implementation Details

• Reads and writes can be parallel.

• Not supported:

• ALTER and SELECT...SAMPLE operations.

• Indexes.
• Zero-copy replication is possible, but not recommended.

##### ZERO-COPY REPLICATION IS NOT READY FOR PRODUCTION

Zero-copy replication is disabled by default in ClickHouse version 22.8 and higher. This feature is not recommended for production use.

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 table function.

Example

1. Suppose we have several files in TSV format with the following URIs on HDFS:
2. 'hdfs://hdfs1:9000/some_dir/some_file_1'
3. 'hdfs://hdfs1:9000/some_dir/some_file_2'
4. 'hdfs://hdfs1:9000/some_dir/some_file_3'
5. 'hdfs://hdfs1:9000/another_dir/some_file_1'
6. 'hdfs://hdfs1:9000/another_dir/some_file_2'
7. 'hdfs://hdfs1:9000/another_dir/some_file_3'
8. There are several ways to make a table consisting of all six files:

CREATE TABLE table_with_range (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/{some,another}_dir/some_file_{1..3}', 'TSV')

Another way:

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):

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:

CREATE TABLE big_table (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/big_dir/file{0..9}{0..9}{0..9}', 'CSV')

https://clickhouse.com/docs/en/engines/table-engines/integrations/hdfs/