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174 lines
7.6 KiB
Markdown
174 lines
7.6 KiB
Markdown
---
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toc_priority: 44
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toc_title: Dictionary Key and Fields
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---
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# Dictionary Key and Fields {#dictionary-key-and-fields}
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The `<structure>` clause describes the dictionary key and fields available for queries.
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XML description:
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``` xml
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<dictionary>
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<structure>
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<id>
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<name>Id</name>
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</id>
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<attribute>
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<!-- Attribute parameters -->
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</attribute>
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...
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</structure>
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</dictionary>
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```
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Attributes are described in the elements:
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- `<id>` — [Key column](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-key).
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- `<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.
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DDL query:
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``` sql
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CREATE DICTIONARY dict_name (
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Id UInt64,
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-- attributes
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)
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PRIMARY KEY Id
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...
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```
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Attributes are described in the query body:
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- `PRIMARY KEY` — [Key column](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-key)
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- `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.
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## Key {#ext_dict_structure-key}
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ClickHouse supports the following types of keys:
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- Numeric key. `UInt64`. Defined in the `<id>` tag or using `PRIMARY KEY` keyword.
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- Composite key. Set of values of different types. Defined in the tag `<key>` or `PRIMARY KEY` keyword.
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An xml structure can contain either `<id>` or `<key>`. DDL-query must contain single `PRIMARY KEY`.
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!!! warning "Warning"
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You must not describe key as an attribute.
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### Numeric Key {#ext_dict-numeric-key}
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Type: `UInt64`.
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Configuration example:
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``` xml
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<id>
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<name>Id</name>
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</id>
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```
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Configuration fields:
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- `name` – The name of the column with keys.
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For DDL-query:
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``` sql
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CREATE DICTIONARY (
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Id UInt64,
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...
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)
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PRIMARY KEY Id
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...
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```
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- `PRIMARY KEY` – The name of the column with keys.
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### Composite Key {#composite-key}
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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`.
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!!! tip "Tip"
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A composite key can consist of a single element. This makes it possible to use a string as the key, for instance.
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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:
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``` xml
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<structure>
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<key>
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<attribute>
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<name>field1</name>
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<type>String</type>
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</attribute>
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<attribute>
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<name>field2</name>
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<type>UInt32</type>
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</attribute>
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...
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</key>
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...
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```
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or
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``` sql
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CREATE DICTIONARY (
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field1 String,
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field2 String
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...
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)
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PRIMARY KEY field1, field2
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...
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```
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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))`.
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## Attributes {#ext_dict_structure-attributes}
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Configuration example:
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``` xml
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<structure>
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...
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<attribute>
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<name>Name</name>
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<type>ClickHouseDataType</type>
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<null_value></null_value>
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<expression>rand64()</expression>
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<hierarchical>true</hierarchical>
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<injective>true</injective>
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<is_object_id>true</is_object_id>
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</attribute>
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</structure>
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```
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or
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``` sql
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CREATE DICTIONARY somename (
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Name ClickHouseDataType DEFAULT '' EXPRESSION rand64() HIERARCHICAL INJECTIVE IS_OBJECT_ID
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)
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```
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Configuration fields:
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| Tag | Description | Required |
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|------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------|
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| `name` | Column name. | Yes |
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| `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 |
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| `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 |
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| `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 |
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| <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 |
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| `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 |
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| `is_object_id` | Flag that shows whether the query is executed for a MongoDB document by `ObjectID`.<br/><br/>Default value: `false`. | No |
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## See Also {#see-also}
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- [Functions for working with external dictionaries](../../../sql-reference/functions/ext-dict-functions.md).
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[Original article](https://clickhouse.tech/docs/en/query_language/dicts/external_dicts_dict_structure/) <!--hide-->
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