Dictionary Key and Fields

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

XML description:

<dictionary>
    <structure>
        <id>
            <name>Id</name>
        </id>
        <attribute>
            <!-- Attribute parameters -->
        </attribute>
        ...
    </structure>
</dictionary>

Attributes are described in the elements:

• <id> — Key column.
• <attribute> — Data column. There can be a multiple number of attributes.

DDL query:

CREATE DICTIONARY dict_name (
    Id UInt64,
    -- attributes
)
PRIMARY KEY Id
...

Attributes are described in the query body:

• PRIMARY KEY — Key column
• AttrName AttrType — Data column. There can be a multiple number of attributes.

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

You must not describe key as an attribute.

Numeric Key

Type: UInt64.

Configuration example:

<id>
    <name>Id</name>
</id>

Configuration fields:

• name – The name of the column with keys.

For DDL-query:

CREATE DICTIONARY (
    Id UInt64,
    ...
)
PRIMARY KEY Id
...

• PRIMARY KEY – 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>
...

or

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

Configuration example:

<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

CREATE DICTIONARY somename (
    Name ClickHouseDataType DEFAULT '' EXPRESSION rand64() HIERARCHICAL INJECTIVE IS_OBJECT_ID
)

Configuration fields:

See Also

• Functions for working with external dictionaries.

Original article