94f86eda79
* Some improvements for introduction/performance.md
* Minor improvements for example_datasets
* Add website/package-lock.json to .gitignore
* YT paragraph was badly outdated and there is no real reason to write a new one
* Use weird introduction article as a starting point for F.A.Q.
* Some refactoring of first half of ya_metrika_task.md
* minor
* Weird docs footer bugfix
* Forgotten redirect
* h/v scrollbars same size in docs
* CLICKHOUSE-3831: introduce security changelog
* A bit more narrow tables on docs front page
* fix flag in ru docs
* Save some space in top level of docs ToC
* Capitalize most words in titles of docs/en/
* more docs scrollbar fixes
* fix incorrect merge
* fix link
* fix switching languages in single page docs mode
* Update mkdocs & mkdocs-material + unminify javascript
* cherrypick 17e18d1ecc
2.4 KiB
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 STATUSquery. - Dictionaries from other sources are updated every time by default.
For MySQL (InnoDB) and ODBC 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>