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Updated fit new structure of en docs.

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Sergei Shtykov 2020-04-08 16:30:02 +03:00
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docs/en/query_language/dicts/external_dicts_dict_lifetime.md
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# 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)
```
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 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.yandex/docs/en/query_language/dicts/external_dicts_dict_lifetime/) <!--hide-->

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docs/en/sql_reference/dictionaries/external_dictionaries/external_dicts_dict_lifetime.md
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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 upgrading the dictionaries, the ClickHouse server applies different logic depending on the type of [source](external_dicts_dict_sources.md):
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.