ClickHouse/docs/zh/query_language/dicts/external_dicts_dict_lifetime.md

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# Dictionary Updates {#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:

``` xml
<dictionary>
    ...
    <lifetime>300</lifetime>
    ...
</dictionary>
```

``` sql
CREATE DICTIONARY (...)
...
LIFETIME(300)
...
```

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

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:

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

or

``` sql
LIFETIME(MIN 300 MAX 360)
```

When upgrading the dictionaries, the ClickHouse server applies different logic depending on the type of [source](external_dicts_dict_sources.md):

-   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](external_dicts_dict_sources.md).

Example of settings:

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

or

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

[Original article](https://clickhouse.tech/docs/en/query_language/dicts/external_dicts_dict_lifetime/) <!--hide-->