Looking at a trace in the "mysql" command-line client can be cumbersome (though the command "pager less" makes it better). An alternative can be to send the trace to a file:

SELECT TRACE INTO DUMPFILE <filename> FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE; 

and then passing this file to some JSON viewer, for example the JsonView Firefox add-on (shows objects in colours, allows object collapsing/expanding). Make sure to use INTO DUMPFILE and not INTO OUTFILE because the latter escapes newlines. Also, have the @@end_marker_in_json variable off: when it's on, the trace is more human-readable but not JSON-compliant.