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119 lines
4.0 KiB
Markdown
119 lines
4.0 KiB
Markdown
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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#ext_dict_structure-key).
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- `<attribute>` — [Data column](external_dicts_dict_structure.md#ext_dict_structure-attributes). There can be a large number of columns.
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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 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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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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### Composite Key
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The key can be a `tuple` from any types of fields. The [layout](external_dicts_dict_layout.md) 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). 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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## 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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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](../../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](../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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`hierarchical` | Hierarchical support. Mirrored to the parent identifier.<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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[Original article](https://clickhouse.yandex/docs/en/query_language/dicts/external_dicts_dict_structure/) <!--hide-->
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