ClickHouse/docs/en/dicts/external_dicts_dict_structure.md

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<a name="dicts-external_dicts_dict_structure"></a>
# Dictionary key and fields
The `<structure>` clause describes the dictionary key and fields available for queries.
Overall structure:
```xml
<dictionary>
<structure>
<id>
<name>Id</name>
</id>
<attribute>
<!-- Attribute parameters -->
</attribute>
...
</structure>
</dictionary>
```
Columns are described in the structure:
- `<id>` [Key column](external_dicts_dict_structure.md#dicts-external_dicts_dict_structure-key).
- `<attribute>` [Data column](external_dicts_dict_structure.md#dicts-external_dicts_dict_structure-attributes). There can be a large number of columns.
<a name="dicts-external_dicts_dict_structure-key"></a>
## Key
ClickHouse supports the following types of keys:
- Numeric key. UInt64. Defined in the tag `<id>` .
- Composite key. Set of values of different types. Defined in the tag `<key>` .
A structure can contain either `<id>` or `<key>` .
<div class="admonition attention">
The key doesn't need to be defined separately in attributes.
</div>
### Numeric key
Format: `UInt64`.
Configuration example:
```xml
<id>
<name>Id</name>
</id>
```
Configuration fields:
- name The name of the column with keys.
### Composite key
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`.
<div class="admonition tip">
A composite key can also consist of a single element, which makes it possible to use a string as the key, for instance.
</div>
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:
```xml
<structure>
<key>
<attribute>
<name>field1</name>
<type>String</type>
</attribute>
<attribute>
<name>field2</name>
<type>UInt32</type>
</attribute>
...
</key>
...
```
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))`.
<a name="dicts-external_dicts_dict_structure-attributes"></a>
## Attributes
Configuration example:
```xml
<structure>
...
<attribute>
<name>Name</name>
<type>Type</type>
<null_value></null_value>
<expression>rand64()</expression>
<hierarchical>true</hierarchical>
<injective>true</injective>
</attribute>
</structure>
```
Configuration fields:
- `name` The column name.
- `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`.
- `null_value` The default value for a non-existing element. In the example, it is an empty string.
- `expression` The attribute can be an expression. The tag is not required.
- `hierarchical` Hierarchical support. Mirrored to the parent identifier. By default, ` false`.
- `injective` Whether the `id -> attribute` image is injective. If ` true`, then you can optimize the ` GROUP BY` clause. By default, `false`.