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111 lines
4.1 KiB
Markdown
111 lines
4.1 KiB
Markdown
---
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slug: /en/sql-reference/table-functions/hdfs
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sidebar_position: 80
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sidebar_label: hdfs
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---
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# hdfs
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Creates a table from files in HDFS. This table function is similar to [url](../../sql-reference/table-functions/url.md) and [file](../../sql-reference/table-functions/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 {#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'`. The strings can contain the `/` symbol.
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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](../../sql-reference/table-functions/remote.md)) table function.
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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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2. 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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3. 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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:::note
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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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:::
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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
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- `_path` — Path to the file. Type: `LowCardinalty(String)`.
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- `_file` — Name of the file. Type: `LowCardinalty(String)`.
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- `_size` — Size of the file in bytes. Type: `Nullable(UInt64)`. If the size is unknown, the value is `NULL`.
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## Storage Settings {#storage-settings}
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- [hdfs_truncate_on_insert](/docs/en/operations/settings/settings.md#hdfs-truncate-on-insert) - allows to truncate file before insert into it. Disabled by default.
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- [hdfs_create_multiple_files](/docs/en/operations/settings/settings.md#hdfs_allow_create_multiple_files) - allows to create a new file on each insert if format has suffix. Disabled by default.
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- [hdfs_skip_empty_files](/docs/en/operations/settings/settings.md#hdfs_skip_empty_files) - allows to skip empty files while reading. Disabled by default.
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- [ignore_access_denied_multidirectory_globs](/docs/en/operations/settings/settings.md#ignore_access_denied_multidirectory_globs) - allows to ignore permission denied errors for multi-directory globs.
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**See Also**
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- [Virtual columns](../../engines/table-engines/index.md#table_engines-virtual_columns)
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