How to Optimize a ClickHouse Query using Projections

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.