Dictionary Updates

ClickHouse periodically updates the dictionaries. The update interval for fully downloaded dictionaries and the invalidation interval for cached dictionaries are defined in the <lifetime> tag in seconds.

Dictionary updates (other than loading for first use) do not block queries. During updates, the old version of a dictionary is used. If an error occurs during an update, the error is written to the server log, and queries continue using the old version of dictionaries.

Example of settings:

<dictionary>
    ...
    <lifetime>300</lifetime>
    ...
</dictionary>

Setting <lifetime> 0</lifetime> prevents updating dictionaries.

You can set a time interval for upgrades, and ClickHouse will choose a uniformly random time within this range. This is necessary in order to distribute the load on the dictionary source when upgrading on a large number of servers.

Example of settings:

<dictionary>
    ...
    <lifetime>
        <min>300</min>
        <max>360</max>
    </lifetime>
    ...
</dictionary>

When upgrading the dictionaries, the ClickHouse server applies different logic depending on the type of source:

• For a text file, it checks the time of modification. If the time differs from the previously recorded time, the dictionary is updated.
• For MyISAM tables, the time of modification is checked using a SHOW TABLE STATUS query.
• Dictionaries from other sources are updated every time by default.

For MySQL (InnoDB), ODBC and ClickHouse sources, you can set up a query that will update the dictionaries only if they really changed, rather than each time. To do this, follow these steps:

• The dictionary table must have a field that always changes when the source data is updated.
• The settings of the source must specify a query that retrieves the changing field. The ClickHouse server interprets the query result as a row, and if this row has changed relative to its previous state, the dictionary is updated. Specify the query in the <invalidate_query> field in the settings for the source.

Example of settings:

<dictionary>
    ...
    <odbc>
      ...
      <invalidate_query>SELECT update_time FROM dictionary_source where id = 1</invalidate_query>
    </odbc>
    ...
</dictionary>

Original article