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>

or

CREATE DICTIONARY (...)
...
LIFETIME(300)
...

Setting <lifetime>0</lifetime> (LIFETIME(0)) prevents dictionaries from updating.

You can set a time interval for updates, 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 updating on a large number of servers.

Example of settings:

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

or

LIFETIME(MIN 300 MAX 360)

If <min>0</min> and <max>0</max>, ClickHouse does not reload the dictionary by timeout.
In this case, ClickHouse can reload the dictionary earlier if the dictionary configuration file was changed or the SYSTEM RELOAD DICTIONARY command was executed.

When updating 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 MySQL source, the time of modification is checked using a SHOW TABLE STATUS query (in case of MySQL 8 you need to disable meta-information caching in MySQL by set global information_schema_stats_expiry=0).
• Dictionaries from other sources are updated every time by default.

For other sources (ODBC, PostgreSQL, ClickHouse, etc), 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>

or

...
SOURCE(ODBC(... invalidate_query 'SELECT update_time FROM dictionary_source where id = 1'))
...

For Cache, ComplexKeyCache, SSDCache, and SSDComplexKeyCache dictionaries both synchronious and asynchronious updates are supported.

It is also possible for Flat, Hashed, ComplexKeyHashed dictionaries to only request data that was changed after the previous update. If update_field is specified as part of the dictionary source configuration, value of the previous update time in seconds will be added to the data request. Depends on source type (Executable, HTTP, MySQL, PostgreSQL, ClickHouse, or ODBC) different logic will be applied to update_field before request data from an external source.

• If the source is HTTP then update_field will be added as a query parameter with the last update time as the parameter value.
• If the source is Executable then update_field will be added as an executable script argument with the last update time as the argument value.
• If the source is ClickHouse, MySQL, PostgreSQL, ODBC there will be an additional part of WHERE, where update_field is compared as greater or equal with the last update time.

If update_field option is set, additional option update_lag can be set. Value of update_lag option is subtracted from previous update time before request updated data.

Example of settings:

<dictionary>
    ...
        <clickhouse>
            ...
            <update_field>added_time</update_field>
            <update_lag>15</update_lag>
        </clickhouse>
    ...
</dictionary>

or

...
SOURCE(CLICKHOUSE(... update_field 'added_time' update_lag 15))
...