Cross-checking a list of users with users on a db - bad idea?

Lets say. If i have 1,000,000 users registered on my system and I have a list of 5开发者_如何学运维00 names.

I would like to crosscheck these names against the number of users on my system to see which names / users are already registered on the db.

Will such a process slow down the app significantly? Or does this sort of thing happen all the time?

Perhaps I can cache the results every 30 mins so I don't have to call it every time.

EDIT >> A bit of clarification:

I forgot to mention that I am trying to udate the list of 500 names. So if 'foobar' and 'joe' is on this list and is also registered in the db, then all I want to do is remove 'foobar' and 'joe' from the list, giving me 498 names.

I don't think it would be suitable to do something like:

User.where('name in (?)', Array('foobar', 'joe'))

I would have do something like:

User.each do |registered_user|
  index = list.index(list.find{ |user| user.screen_name.downcase == registered_user.screen_name.downcase })
  list.delete_at(index) if index
end

filtered_list = list

Is above code, overkill?

Any modern relational DB worth it's salt should easily be able to process 500 checks against the DB without affecting other queries, provided that your tables are structured properly (unique keys, indexes on frequently searched fields, etc)

If you do it cleverly, it can be very fast. Let the database do the work.

SELECT * FROM USERS WHERE Name IN ( <generated list of your 500 user name candidates>)

The returned list will be the ones of the 500 which already exist in the database. Such as call will be fast, at most a few seconds on a slow database.

Cheers, Daniel