System Tables and a Window into the Internals of ClickHouse

The system tables in ClickHouse provide valuable insights into the internal operations of the server, including database settings, query execution, and system performance. System tables can be used to debug issues, understand cluster usage, and identify potential bottlenecks. The most commonly used system tables include `system.databases`, `system.processes`, `system.query_log`, `system.settings`, `system.part_log`, `system.merges`, and `system.moves`. These tables offer a wealth of information about the server state, including metrics, events, and errors, allowing users to gain a deeper understanding of the system and optimize queries for better performance. By querying these tables using standard SQL syntax, users can retrieve rows from all nodes in a cluster with shards and replicas, providing a comprehensive view of the system's performance and usage.