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118 lines
3.4 KiB
Markdown
118 lines
3.4 KiB
Markdown
<a name="dicts-external_dicts_dict_structure"></a>
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# Dictionary key and fields
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The `<structure>` clause describes the dictionary key and fields available for queries.
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Overall structure:
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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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Columns are described in the structure:
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- `<id>` - [key column](external_dicts_dict_structure.md#dicts-external_dicts_dict_structure-key).
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- `<attribute>` - [data column](external_dicts_dict_structure.md#dicts-external_dicts_dict_structure-attributes). There can be a large number of columns.
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<a name="dicts-external_dicts_dict_structure-key"></a>
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## Key
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ClickHouse supports the following types of keys:
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- Numeric key. UInt64. Defined in the tag `<id>` .
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- Composite key. Set of values of different types. Defined in the tag `<key>` .
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A structure can contain either `<id>` or `<key>` .
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!!! warning
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The key doesn't need to be defined separately in attributes.
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### Numeric key
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Format: `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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### Composite key
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The key can be a `tuple` from any types of fields. The [layout](external_dicts_dict_layout.md#dicts-external_dicts_dict_layout) in this case must be `complex_key_hashed` or `complex_key_cache`.
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!!! 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](external_dicts_dict_structure.md#dicts-external_dicts_dict_structure-attributes). 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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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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<a name="dicts-external_dicts_dict_structure-attributes"></a>
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## 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>Type</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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Configuration fields:
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- `name` – The column name.
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- `type` – The column type. Sets the method for interpreting data in the source. For example, for MySQL, the field might be `TEXT`, `VARCHAR`, or `BLOB` in the source table, but it can be uploaded as `String`.
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- `null_value` – The default value for a non-existing element. In the example, it is an empty string.
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- `expression` – The attribute can be an expression. The tag is not required.
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- `hierarchical` – Hierarchical support. Mirrored to the parent identifier. By default, ` false`.
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- `injective` – Whether the `id -> attribute` image is injective. If ` true`, then you can optimize the ` GROUP BY` clause. By default, `false`.
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- `is_object_id` – Whether the query is executed for a MongoDB document by `ObjectID`.
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