debugging how query plans are made

[Zardosht Kasheff]

Hello all,

In MySQL and/or MariaDB, are there any hooks or methods one can use to
learn why a query plan is chosen over alternatives? We sometimes
notice that a different query plan is chosen for us than InnoDB (which
may be fine, given we are a different storage engine), and we would
like to better understand why. Is there any debug parameter that gets
MySQL to output decisions made in the optimizer?

Thanks
-Zardosht