How to Optimize a ClickHouse Query using Projections

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Projections are a query-optimization feature of MergeTree tables in ClickHouse that store data in a different format than the defined sort order. Useful use cases include the ability to sort the data in a different order than the primary key, and also to pre-aggregate columns. One nice benefit of projections over materialized views is that a projection does not require a separate table, so users simply query the original table and ClickHouse decides at query time if a projection can improve the performance.
Reference links:
ClickHouse Quick Start Documentation: clickhouse.com...
ClickHouse Academy: clickhouse.com...
Official ClickHouse Certification: clickhouse.com...

Опубликовано:

28 сен 2024

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Комментарии : 8

@101graffhead Месяц назад

do projections don't get used in replicatedMergeTree? let's say if i create a projection on table like select * order by , will it be equivalent to adding that column in order by key in that table, if that didn't help, will creating a new table with my desired primary key help my queries, (i kind of chose suboptimal primary keys when creating the table but now we have ton of data and migration seem scary.), another stupid question, i am assume joins cannot be be used in projection statement. thanks.

@developerjas Год назад

That’s useful

@yes-ni1od 11 месяцев назад

Projections are awesomesauce, and Clickhouse is awesomer, native column compression with zstd

@DmytroMytko 4 месяца назад

Sorry for the probably stupid question, but if we just add skip index to town, will it help with the 1st case to speed up it? Can it be an alternative to projection or what is the significant difference between them? Thank you!