async_load_* server settings

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async_load_databases

• If true all non-system databases with Ordinary, Atomic and Replicated engine will be loaded asynchronously after the ClickHouse server start up. See system.asynchronous_loader table, tables_loader_background_pool_size and tables_loader_foreground_pool_size server settings. Any query that tries to access a table, that is not yet loaded, will wait for exactly this table to be started up. If load job fails, query will rethrow an error (instead of shutting down the whole server in case of async_load_databases = false). The table that is waited for by at least one query will be loaded with higher priority. DDL queries on a database will wait for exactly that database to be started up. Also consider setting a limit max_waiting_queries for the total number of waiting queries.
• If false, all databases are loaded when the server starts.

<async_load_databases>true</async_load_databases>

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async_load_system_database

• If set to true, all system databases with Ordinary, Atomic, and Replicated engines will be loaded asynchronously after the ClickHouse server starts. See system.asynchronous_loader table, tables_loader_background_pool_size and tables_loader_foreground_pool_size server settings. Any query that tries to access a system table, that is not yet loaded, will wait for exactly this table to be started up. The table that is waited for by at least one query will be loaded with higher priority. Also consider setting the max_waiting_queries setting to limit the total number of waiting queries.
• If set to false, system database loads before server start.

<async_load_system_database>true</async_load_system_database>