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98 lines
3.2 KiB
Markdown
98 lines
3.2 KiB
Markdown
# hdfs {#hdfs}
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Creates a table from files in HDFS. This table function is similar to [url](url.md) and [file](file.md) ones.
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``` sql
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hdfs(URI, format, structure)
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```
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**Input parameters**
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- `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.
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- `format` — The [format](../../interfaces/formats.md#formats) of the file.
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- `structure` — Structure of the table. Format `'column1_name column1_type, column2_name column2_type, ...'`.
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**Returned value**
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A table with the specified structure for reading or writing data in the specified file.
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**Example**
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Table from `hdfs://hdfs1:9000/test` and selection of the first two rows from it:
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``` sql
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SELECT *
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FROM hdfs('hdfs://hdfs1:9000/test', 'TSV', 'column1 UInt32, column2 UInt32, column3 UInt32')
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LIMIT 2
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```
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``` text
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┌─column1─┬─column2─┬─column3─┐
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│ 1 │ 2 │ 3 │
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│ 3 │ 2 │ 1 │
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└─────────┴─────────┴─────────┘
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```
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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 (not only suffix or prefix).
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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 that we have several files with 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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1. Query the amount of rows in these files:
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<!-- -->
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``` sql
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SELECT count(*)
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FROM hdfs('hdfs://hdfs1:9000/{some,another}_dir/some_file_{1..3}', 'TSV', 'name String, value UInt32')
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```
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1. Query the amount of rows in all files of these two directories:
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<!-- -->
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``` sql
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SELECT count(*)
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FROM hdfs('hdfs://hdfs1:9000/{some,another}_dir/*', 'TSV', 'name String, value UInt32')
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```
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!!! warning "Warning"
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If your 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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Query the data from files named `file000`, `file001`, … , `file999`:
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``` sql
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SELECT count(*)
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FROM hdfs('hdfs://hdfs1:9000/big_dir/file{0..9}{0..9}{0..9}', 'CSV', 'name String, value UInt32')
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```
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## Virtual Columns {#virtual-columns}
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- `_path` — Path to the file.
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- `_file` — Name of the file.
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**See Also**
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- [Virtual columns](https://clickhouse.tech/docs/en/operations/table_engines/#table_engines-virtual_columns)
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[Original article](https://clickhouse.tech/docs/en/query_language/table_functions/hdfs/) <!--hide-->
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