Are values indexed that are inserted before the column in mysql table is indexed

Question

So lets say I have column email in table user. And I created the table user email column is not neither foreign key , or unique or whatever that is indexed. After that I'm adding index to the column email. Do the values, before adding index in the email column, have indexed or not?

Answer 1

Yes.

When you create an index (be it a simple index, or a special case like unique/primary/...), the database will index all values that are already in the table .

This is why for a small (empty) table, creating an index can be fast, but for a large table, it can take much longer.