Creating an external dictionary in ClickHouse from a URL involves retrieving data from a remote source and utilizing it as a dictionary within your ClickHouse processes. ClickHouse supports various dictionary sources and configurations, and using an HTTP URL as a data source is one of the flexible methods. This facilitates dynamic data retrieval and minimizes manual maintenance by fetching the expected output automatically through HTTP requests.

Understanding ClickHouse Dictionaries

In ClickHouse, dictionaries are used to enhance query performance, particularly for join operations. They provide an efficient way to map key values to complex data structures. Datasets from dictionaries can be joined with table data during queries, reducing computation time significantly compared to traditional joins.

Steps to Create an External Dictionary from a URL

To create an external dictionary in ClickHouse using a URL, follow these detailed steps:

1. Define the Dictionary Structure

The dictionary structure must be defined in XML. ClickHouse requires specific fields such as id, key, and attribute to parse and use the dictionary effectively:

1<yandex>
2    <dictionary>
3        <name>url_based_dict</name>
4        <source>
5            <http>
6                <url>http://your-data-source-url.com/dictionary-data</url>
7                <format>TabSeparated</format>
8            </http>
9        </source>
10        <layout>
11            <flat/>
12        </layout>
13        <structure>
14            <id>
15                <name>id</name>
16                <type>UInt32</type>
17            </id>
18            <key>
19                <attribute>
20                    <name>key_column</name>
21                    <type>String</type>
22                </attribute>
23            </key>
24            <attribute>
25                <name>value_attribute</name>
26                <type>String</type>
27                <null_value></null_value>
28            </attribute>
29        </structure>
30        <lifetime>
31            <min>600</min>
32            <max>3600</max>
33        </lifetime>
34    </dictionary>
35</yandex>

Explanation:

• Name: The name is the identifier of the dictionary within ClickHouse.
• Source: Specifies the source type (http) and the URL hosting the dictionary data. The format field should match the data format (TabSeparated, CSV, etc.)
• Structure: Defines id, key, and attributes with explicitly declared types. Types must match the column data types.
• Layout: Describes how dictionaries are stored and accessed. Options like <flat/>, <hashed/> influence performance.
• Lifetime: Defines cache durations. min and max control dictionary refresh timings to consider the newest data updates.

Example Configuration

An example configuration file can be placed in the user_dictionaries directory, usually located in /etc/clickhouse-server/dictionaries/:

1cat > /etc/clickhouse-server/dictionaries/url_dictionary.xml <<EOF
2<yandex>
3    <dictionary>
4        <name>url_based_dict</name>
5        <source>
6            <http>
7                <url>http://your-data-source-url.com/dictionary-data</url>
8                <format>JSONEachRow</format>
9            </http>
10        </source>
11        <layout>
12            <flat/>
13        </layout>
14        <structure>
15            <id>
16                <name>id</name>
17                <type>UInt32</type>
18            </id>
19            <key>
20                <attribute>
21                    <name>key_column</name>
22                    <type>String</type>
23                </attribute>
24            </key>
25            <attribute>
26                <name>value_attribute</name>
27                <type>String</type>
28                <null_value></null_value>
29            </attribute>
30        </structure>
31        <lifetime>
32            <min>600</min>
33            <max>3600</max>
34        </lifetime>
35    </dictionary>
36</yandex>
37EOF

Querying the Dictionary

Once configured, you can query the dictionary in SQL to enhance data operations:

SELECT dictGetString('url_based_dict', 'value_attribute', key_column) 
FROM some_table;

Summary Table

Additional Notes

1. Data Format: Ensure that the provided data format aligns with that of the response data from the URL.
2. Security: Consider data security, especially when retrieving data over HTTP. Use secured HTTPS where possible.
3. Testing: It’s advisable to test your dictionary setup with a small dataset before applying it to production environments.
4. Handling Updates: Regularly verify the source data for updates that require dictionary synchronization to maintain data consistency.

By following these steps, you can efficiently set up an external dictionary in ClickHouse using data fetched directly from a URL, enabling dynamic data operations and optimized query performance.