Student: what are indexes on mysql tables?

Teacher: I don't know, nor do I recommend it's usage.

It's good for queries. But after a million rows (or so) you should try partitioning it.

How can you recommend not to do something if you don't know what it is?! Indexing (correctly) is important!

I could not keep my mouth shut after that, and I got on the bad side of the teacher because of correcting him...

Also you don't have to wait for a million rows, a few badly written queries are enough to degrade the preformance.