CollapsingMergeTree Vs ReplacingMergeTree

[Mai Abdel-Qader]

Hello,

What is the most suitable engine of table that I keep updates its column depending on primary key? Some updated columns are strings and the others numeric.

Should I go with CollapsingMergeTree or ReplacingMergeTree? 

And what is the frequency of updates is high, should I go with VersionedCollapsingMergeTree?

Thanks a lot,

Mai

[Denis Zhuravlev]

OLAP data is mostly immutable. CH does not have engines which allows frequent updates.

Check external dictionaries for mutable dimensions.

[Mai Abdel-Qader]

What if the updates are not that frequent? With which engine should I go?

Are you aware of specific external dictionaries that might help me to handle mutable dimensions?

[Denis Zhuravlev]

>What if the updates are not that frequent? With which engine should I go?

It depends of your requirements. Try Replacing first.

>Are you aware of specific external dictionaries that might help me to handle mutable dimensions?

They have different features. Hashed is the most commonly used.

https://clickhouse.yandex/docs/en/query_language/dicts/external_dicts_dict_layout/#dicts-external_dicts_dict_layout-hashed

[Mai Abdel-Qader]

Thanks a lot.

In general what is the difference between CollapsingMergeTree and ReplacingMergeTree?