Explain [extended] lays out the optimiser's execution plan for a query. Since version 5.6.3, turning on the optimiser trace facility for a session can show alternative plans that the optimiser considered ...

set optimizer_trace="enabled=on";

The next query is written to information_schema.optimizer_trace.query, the optimizer trace is written to information_schema.optimizer_trace.trace.

If information_schema.optimizer_trace.missing_bytes_beyond_max_mem_size is not empty, increase available memory for the trace ...

set optimizer_trace_max_mem_size=1000000;

and run the query again.

Remember to turn tracing off when you're done.