ClickHouse/docs/en/operations/system-tables/columns.md

91 lines
5.5 KiB
Markdown
Raw Normal View History

2022-08-28 13:53:52 +00:00
---
2022-08-28 13:58:27 +00:00
slug: /en/operations/system-tables/columns
2022-08-28 13:53:52 +00:00
---
2022-06-02 10:55:18 +00:00
# columns
Contains information about columns in all the tables.
You can use this table to get information similar to the [DESCRIBE TABLE](../../sql-reference/statements/describe-table.md) query, but for multiple tables at once.
2021-07-29 15:20:55 +00:00
Columns from [temporary tables](../../sql-reference/statements/create/table.md#temporary-tables) are visible in the `system.columns` only in those session where they have been created. They are shown with the empty `database` field.
2021-04-17 22:47:43 +00:00
The `system.columns` table contains the following columns (the column type is shown in brackets):
- `database` ([String](../../sql-reference/data-types/string.md)) — Database name.
- `table` ([String](../../sql-reference/data-types/string.md)) — Table name.
- `name` ([String](../../sql-reference/data-types/string.md)) — Column name.
2023-10-11 14:34:28 +00:00
- `type` ([String](../../sql-reference/data-types/string.md)) — Column type.
- `position` ([UInt64](../../sql-reference/data-types/int-uint.md)) — Ordinal position of a column in a table starting with 1.
- `default_kind` ([String](../../sql-reference/data-types/string.md)) — Expression type (`DEFAULT`, `MATERIALIZED`, `ALIAS`) for the default value, or an empty string if it is not defined.
- `default_expression` ([String](../../sql-reference/data-types/string.md)) — Expression for the default value, or an empty string if it is not defined.
- `data_compressed_bytes` ([UInt64](../../sql-reference/data-types/int-uint.md)) — The size of compressed data, in bytes.
- `data_uncompressed_bytes` ([UInt64](../../sql-reference/data-types/int-uint.md)) — The size of decompressed data, in bytes.
- `marks_bytes` ([UInt64](../../sql-reference/data-types/int-uint.md)) — The size of marks, in bytes.
- `comment` ([String](../../sql-reference/data-types/string.md)) — Comment on the column, or an empty string if it is not defined.
- `is_in_partition_key` ([UInt8](../../sql-reference/data-types/int-uint.md)) — Flag that indicates whether the column is in the partition expression.
- `is_in_sorting_key` ([UInt8](../../sql-reference/data-types/int-uint.md)) — Flag that indicates whether the column is in the sorting key expression.
- `is_in_primary_key` ([UInt8](../../sql-reference/data-types/int-uint.md)) — Flag that indicates whether the column is in the primary key expression.
- `is_in_sampling_key` ([UInt8](../../sql-reference/data-types/int-uint.md)) — Flag that indicates whether the column is in the sampling key expression.
- `compression_codec` ([String](../../sql-reference/data-types/string.md)) — Compression codec name.
- `character_octet_length` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — Maximum length in bytes for binary data, character data, or text data and images. In ClickHouse makes sense only for `FixedString` data type. Otherwise, the `NULL` value is returned.
2023-06-02 11:30:05 +00:00
- `numeric_precision` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — Accuracy of approximate numeric data, exact numeric data, integer data, or monetary data. In ClickHouse it is bit width for integer types and decimal precision for `Decimal` types. Otherwise, the `NULL` value is returned.
- `numeric_precision_radix` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — The base of the number system is the accuracy of approximate numeric data, exact numeric data, integer data or monetary data. In ClickHouse it's 2 for integer types and 10 for `Decimal` types. Otherwise, the `NULL` value is returned.
- `numeric_scale` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — The scale of approximate numeric data, exact numeric data, integer data, or monetary data. In ClickHouse makes sense only for `Decimal` types. Otherwise, the `NULL` value is returned.
- `datetime_precision` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — Decimal precision of `DateTime64` data type. For other data types, the `NULL` value is returned.
2020-12-03 19:15:29 +00:00
**Example**
```sql
SELECT * FROM system.columns LIMIT 2 FORMAT Vertical;
2020-12-03 19:15:29 +00:00
```
```text
Row 1:
──────
database: INFORMATION_SCHEMA
table: COLUMNS
name: table_catalog
2020-12-03 19:15:29 +00:00
type: String
position: 1
2021-07-29 15:27:50 +00:00
default_kind:
default_expression:
2020-12-03 19:15:29 +00:00
data_compressed_bytes: 0
data_uncompressed_bytes: 0
marks_bytes: 0
2021-07-29 15:27:50 +00:00
comment:
2020-12-03 19:15:29 +00:00
is_in_partition_key: 0
is_in_sorting_key: 0
is_in_primary_key: 0
is_in_sampling_key: 0
2021-07-29 15:27:50 +00:00
compression_codec:
character_octet_length: ᴺᵁᴸᴸ
numeric_precision: ᴺᵁᴸᴸ
numeric_precision_radix: ᴺᵁᴸᴸ
numeric_scale: ᴺᵁᴸᴸ
datetime_precision: ᴺᵁᴸᴸ
2020-12-03 19:15:29 +00:00
Row 2:
──────
database: INFORMATION_SCHEMA
table: COLUMNS
name: table_schema
type: String
position: 2
2021-07-29 15:27:50 +00:00
default_kind:
default_expression:
2020-12-03 19:15:29 +00:00
data_compressed_bytes: 0
data_uncompressed_bytes: 0
marks_bytes: 0
2021-07-29 15:27:50 +00:00
comment:
2020-12-03 19:15:29 +00:00
is_in_partition_key: 0
is_in_sorting_key: 0
is_in_primary_key: 0
is_in_sampling_key: 0
2021-07-29 15:27:50 +00:00
compression_codec:
character_octet_length: ᴺᵁᴸᴸ
numeric_precision: ᴺᵁᴸᴸ
numeric_precision_radix: ᴺᵁᴸᴸ
numeric_scale: ᴺᵁᴸᴸ
datetime_precision: ᴺᵁᴸᴸ
2020-12-03 19:15:29 +00:00
```