show variables like 'optimizer_trace';
set session optimizer_trace="enabled=on",end_markers_in_json=on;
set OPTIMIZER_TRACE_MAX_MEM_SIZE=1000000;
select host,user,plugin from user;
SELECT trace FROM information_schema.OPTIMIZER_TRACE;
SELECT TRACE INTO DUMPFILE "/tmp/test.trace" FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
注意:(如果没有控制台权限,或直接交由运维,让他把该 trace 文件,输出给你就行了。)。
注意:不设置优化器最大容量的话,可能会导致优化器返回的结果不全。