NOTE
MySQL System Databases
Purpose of mysql, information_schema, performance_schema, and sys metadata/diagnostic schemas.
This is a historical learning note and may contain outdated or incomplete understanding.
MySQL includes system schemas for server metadata and operation. mysql stores internal privilege/configuration metadata, information_schema exposes relational metadata views, performance_schema captures runtime instrumentation, and sys provides higher-level diagnostic views built on that instrumentation.
Exact objects vary by MySQL version. Query these schemas through documented interfaces rather than treating underlying internal tables as stable application data.