ClickHouse/docs/en/query_language/dicts/external_dicts_dict_structure.md

Dictionary Key and Fields

The <structure> clause describes the dictionary key and fields available for queries.

Overall structure:

<dictionary>
    <structure>
        <id>
            <name>Id</name>
        </id>

        <attribute>
            <!-- Attribute parameters -->
        </attribute>

        ...

    </structure>
</dictionary>

Columns are described in the structure:

• <id> - key column.
• <attribute> - data column. There can be a large number of columns.

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> .

!!! warning The key doesn't need to be defined separately in attributes.

Numeric Key

Format: UInt64.

Configuration example:

<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 in this case must be complex_key_hashed or complex_key_cache.

!!! 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. Example:

<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)).

Attributes

Configuration example:

<structure>
    ...
    <attribute>
        <name>Name</name>
        <type>Type</type>
        <null_value></null_value>
        <expression>rand64()</expression>
        <hierarchical>true</hierarchical>
        <injective>true</injective>
        <is_object_id>true</is_object_id>
    </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.
• is_object_id – Whether the query is executed for a MongoDB document by ObjectID.

Original article