Examining a trace in the mysql command-line client can be made less difficult using the pager less command (or your operating platform's equivalent). An alternative can be to send the trace to a file, similarly to what is shown here:

SELECT TRACE INTO DUMPFILE file
FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;

You can then pass this file to a JSON-aware text editor or other viewer, such as the JsonView add-on for Firefox and Chrome, which shows objects in color and allows objects to be expanded or collapsed.

INTO DUMPFILE is preferable to INTO OUTFILE for this purpose, since the latter escapes newlines. As noted previously, you should ensure that end_markers_in_json is OFFwhen executing the SELECT INTO statement, so that the output is valid JSON.