SELECT @@global.optimizer_trace_limit; @@global.optimizer_trace_limit 1 SET @@global.optimizer_trace_limit=5; show global variables like 'optimizer_trace_limit'; Variable_name Value optimizer_trace_limit 5 select * from information_schema.global_variables where variable_name='optimizer_trace_limit'; VARIABLE_NAME VARIABLE_VALUE OPTIMIZER_TRACE_LIMIT 5 select @@session.optimizer_trace_limit; @@session.optimizer_trace_limit 1 show session variables like 'optimizer_trace_limit'; Variable_name Value optimizer_trace_limit 1 select * from information_schema.session_variables where variable_name='optimizer_trace_limit'; VARIABLE_NAME VARIABLE_VALUE OPTIMIZER_TRACE_LIMIT 1 set session optimizer_trace_limit=10; select @@session.optimizer_trace_limit; @@session.optimizer_trace_limit 10 show session variables like 'optimizer_trace_limit'; Variable_name Value optimizer_trace_limit 10 set session optimizer_trace_limit=0; select @@session.optimizer_trace_limit; @@session.optimizer_trace_limit 0 show session variables like 'optimizer_trace_limit'; Variable_name Value optimizer_trace_limit 0 set session optimizer_trace_limit=-100; Warnings: Warning 1292 Truncated incorrect optimizer_trace_limit value: '-100' select @@session.optimizer_trace_limit; @@session.optimizer_trace_limit 0 show session variables like 'optimizer_trace_limit'; Variable_name Value optimizer_trace_limit 0 set session optimizer_trace_limit=default; select @@session.optimizer_trace_limit; @@session.optimizer_trace_limit 5 show session variables like 'optimizer_trace_limit'; Variable_name Value optimizer_trace_limit 5 set global optimizer_trace_limit=default; select @@global.optimizer_trace_limit; @@global.optimizer_trace_limit 1 show global variables like 'optimizer_trace_limit'; Variable_name Value optimizer_trace_limit 1 set optimizer_trace_limit=1.1; ERROR 42000: Incorrect argument type to variable 'optimizer_trace_limit' set optimizer_trace_limit="foobar"; ERROR 42000: Incorrect argument type to variable 'optimizer_trace_limit'