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61 lines
2.4 KiB
Markdown
61 lines
2.4 KiB
Markdown
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# Dictionary Updates
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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.
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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.
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Example of settings:
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```xml
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<dictionary>
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...
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<lifetime>300</lifetime>
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...
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</dictionary>
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```
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Setting ` <lifetime> 0</lifetime> ` prevents updating dictionaries.
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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.
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Example of settings:
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```xml
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<dictionary>
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...
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<lifetime>
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<min>300</min>
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<max>360</max>
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</lifetime>
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...
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</dictionary>
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```
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When upgrading the dictionaries, the ClickHouse server applies different logic depending on the type of [ source](external_dicts_dict_sources.md):
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- For a text file, it checks the time of modification. If the time differs from the previously recorded time, the dictionary is updated.
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- For MyISAM tables, the time of modification is checked using a `SHOW TABLE STATUS` query.
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- Dictionaries from other sources are updated every time by default.
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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:
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- The dictionary table must have a field that always changes when the source data is updated.
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- 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).
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Example of settings:
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```xml
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<dictionary>
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...
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<odbc>
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...
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<invalidate_query>SELECT update_time FROM dictionary_source where id = 1</invalidate_query>
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</odbc>
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...
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</dictionary>
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```
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[Original article](https://clickhouse.yandex/docs/en/query_language/dicts/external_dicts_dict_lifetime/) <!--hide-->
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