MySQL 8.4 Release Notes
To perform optimizer tracing entails the following steps:
Enable tracing by executing
SEToptimizer_trace="enabled=ON".Execute the statement to be traced. See Section 10.15.3, “Traceable Statements”, for a listing of statements which can be traced.
Examine the contents of the
INFORMATION_SCHEMA.OPTIMIZER_TRACEtable.To examine traces for multiple queries, repeat the previous two steps as needed.
To disable tracing after you have finished, execute
SET optimizer_trace="enabled=OFF".
You can trace only statements which are executed within the current session; you cannot see traces from other sessions.