ClickHouse/docs/en/database_engines/lazy.md

Lazy

Works like Ordinary, but keeps tables in RAM only expiration_time_in_seconds seconds after last access. Can be used only with *Log tables.

It's optimized for storing many small *Log tables, for which there is a long time interval between accesses.

Creating a Database

CREATE DATABASE testlazy ENGINE = Lazy(expiration_time_in_seconds);

Original article