mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-11-17 13:13:36 +00:00
7170f3c534
* prefer relative links from root * wip * split aggregate function reference * split system tables
174 lines
7.6 KiB
Markdown
174 lines
7.6 KiB
Markdown
---
|
||
toc_priority: 44
|
||
toc_title: Dictionary Key and Fields
|
||
---
|
||
|
||
# Dictionary Key and Fields {#dictionary-key-and-fields}
|
||
|
||
The `<structure>` clause describes the dictionary key and fields available for queries.
|
||
|
||
XML description:
|
||
|
||
``` xml
|
||
<dictionary>
|
||
<structure>
|
||
<id>
|
||
<name>Id</name>
|
||
</id>
|
||
|
||
<attribute>
|
||
<!-- Attribute parameters -->
|
||
</attribute>
|
||
|
||
...
|
||
|
||
</structure>
|
||
</dictionary>
|
||
```
|
||
|
||
Attributes are described in the elements:
|
||
|
||
- `<id>` — [Key column](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-key).
|
||
- `<attribute>` — [Data column](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-attributes). There can be a multiple number of attributes.
|
||
|
||
DDL query:
|
||
|
||
``` sql
|
||
CREATE DICTIONARY dict_name (
|
||
Id UInt64,
|
||
-- attributes
|
||
)
|
||
PRIMARY KEY Id
|
||
...
|
||
```
|
||
|
||
Attributes are described in the query body:
|
||
|
||
- `PRIMARY KEY` — [Key column](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-key)
|
||
- `AttrName AttrType` — [Data column](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-attributes). There can be a multiple number of attributes.
|
||
|
||
## Key {#ext_dict_structure-key}
|
||
|
||
ClickHouse supports the following types of keys:
|
||
|
||
- Numeric key. `UInt64`. Defined in the `<id>` tag or using `PRIMARY KEY` keyword.
|
||
- Composite key. Set of values of different types. Defined in the tag `<key>` or `PRIMARY KEY` keyword.
|
||
|
||
An xml structure can contain either `<id>` or `<key>`. DDL-query must contain single `PRIMARY KEY`.
|
||
|
||
!!! warning "Warning"
|
||
You must not describe key as an attribute.
|
||
|
||
### Numeric Key {#ext_dict-numeric-key}
|
||
|
||
Type: `UInt64`.
|
||
|
||
Configuration example:
|
||
|
||
``` xml
|
||
<id>
|
||
<name>Id</name>
|
||
</id>
|
||
```
|
||
|
||
Configuration fields:
|
||
|
||
- `name` – The name of the column with keys.
|
||
|
||
For DDL-query:
|
||
|
||
``` sql
|
||
CREATE DICTIONARY (
|
||
Id UInt64,
|
||
...
|
||
)
|
||
PRIMARY KEY Id
|
||
...
|
||
```
|
||
|
||
- `PRIMARY KEY` – The name of the column with keys.
|
||
|
||
### Composite Key {#composite-key}
|
||
|
||
The key can be a `tuple` from any types of fields. The [layout](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-layout.md) in this case must be `complex_key_hashed` or `complex_key_cache`.
|
||
|
||
!!! tip "Tip"
|
||
A composite key can consist of a single element. This makes it possible to use a string as the key, for instance.
|
||
|
||
The key structure is set in the element `<key>`. Key fields are specified in the same format as the dictionary [attributes](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md). Example:
|
||
|
||
``` xml
|
||
<structure>
|
||
<key>
|
||
<attribute>
|
||
<name>field1</name>
|
||
<type>String</type>
|
||
</attribute>
|
||
<attribute>
|
||
<name>field2</name>
|
||
<type>UInt32</type>
|
||
</attribute>
|
||
...
|
||
</key>
|
||
...
|
||
```
|
||
|
||
or
|
||
|
||
``` sql
|
||
CREATE DICTIONARY (
|
||
field1 String,
|
||
field2 String
|
||
...
|
||
)
|
||
PRIMARY KEY field1, field2
|
||
...
|
||
```
|
||
|
||
For a query to the `dictGet*` function, a tuple is passed as the key. Example: `dictGetString('dict_name', 'attr_name', tuple('string for field1', num_for_field2))`.
|
||
|
||
## Attributes {#ext_dict_structure-attributes}
|
||
|
||
Configuration example:
|
||
|
||
``` xml
|
||
<structure>
|
||
...
|
||
<attribute>
|
||
<name>Name</name>
|
||
<type>ClickHouseDataType</type>
|
||
<null_value></null_value>
|
||
<expression>rand64()</expression>
|
||
<hierarchical>true</hierarchical>
|
||
<injective>true</injective>
|
||
<is_object_id>true</is_object_id>
|
||
</attribute>
|
||
</structure>
|
||
```
|
||
|
||
or
|
||
|
||
``` sql
|
||
CREATE DICTIONARY somename (
|
||
Name ClickHouseDataType DEFAULT '' EXPRESSION rand64() HIERARCHICAL INJECTIVE IS_OBJECT_ID
|
||
)
|
||
```
|
||
|
||
Configuration fields:
|
||
|
||
| Tag | Description | Required |
|
||
|------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------|
|
||
| `name` | Column name. | Yes |
|
||
| `type` | ClickHouse data type.<br/>ClickHouse tries to cast value from dictionary to the specified data type. For example, for MySQL, the field might be `TEXT`, `VARCHAR`, or `BLOB` in the MySQL source table, but it can be uploaded as `String` in ClickHouse.<br/>[Nullable](../../../sql-reference/data-types/nullable.md) is not supported. | Yes |
|
||
| `null_value` | Default value for a non-existing element.<br/>In the example, it is an empty string. You cannot use `NULL` in this field. | Yes |
|
||
| `expression` | [Expression](../../../sql-reference/syntax.md#syntax-expressions) that ClickHouse executes on the value.<br/>The expression can be a column name in the remote SQL database. Thus, you can use it to create an alias for the remote column.<br/><br/>Default value: no expression. | No |
|
||
| <a name="hierarchical-dict-attr"></a> `hierarchical` | If `true`, the attribute contains the value of a parent key for the current key. See [Hierarchical Dictionaries](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-hierarchical.md).<br/><br/>Default value: `false`. | No |
|
||
| `injective` | Flag that shows whether the `id -> attribute` image is [injective](https://en.wikipedia.org/wiki/Injective_function).<br/>If `true`, ClickHouse can automatically place after the `GROUP BY` clause the requests to dictionaries with injection. Usually it significantly reduces the amount of such requests.<br/><br/>Default value: `false`. | No |
|
||
| `is_object_id` | Flag that shows whether the query is executed for a MongoDB document by `ObjectID`.<br/><br/>Default value: `false`. | No |
|
||
|
||
## See Also {#see-also}
|
||
|
||
- [Functions for working with external dictionaries](../../../sql-reference/functions/ext-dict-functions.md).
|
||
|
||
[Original article](https://clickhouse.tech/docs/en/query_language/dicts/external_dicts_dict_structure/) <!--hide-->
|