Shouldn't the first line have index on first_name + last_name and last_name + first_name instead of 2 times first_name + last_name?

Dear sirs,

Can you perhaps explain how the double braces work in bash?

I'm having a hard time to understand it properly.

Thank you in advance!

Is it the shortest way to write a real one-line function?

I think, there should be test like that one:
[1].sameStructureAs([[1]]) // false
It's possible to pass a solution that doesn't cover such conditions.

There is no formula here. I just noticed a ratio between values and used that.

The solution can't overcome a time limit without an index. Therefore, an index is not useless here.

Which formula is applied in this kata?

Index did nothing in this case, you scanned too much rows to bet useful.
Don't use it mindlessly at least count records before adding it to see if there is prospect for index use.

Less than 7 seconds for all tests.