On Tuesday, 11 July 2017 09:33:05 UTC-4, Ralph Shnelvar wrote:
>
> Let's say I expect to have a billion users in my Postgres users table.
>
> I want to make sure the first user gets to set it's role attribute to enum
> :admin as the default when the record is created and all other users are
> enum :user.
>
> Of course a billion users is ridiculous. I'm just trying to understand
> what the, uh, best way to see if a Postgres table is empty without actually
> having to load it.
>
I suspect `User.exists?` is going to be the lightest possible option.
`User.count == 0` will also work, but I know there are some DBs where that
operation can be expensive on large tables.
This is still going to do a query every time it checks, so if that's too
much load you could cache the result:
class User < ActiveRecord::Base
def self.any_users?
@any_users ||= exists?
end
end
Then your role-defaulting code can check `self.class.any_users?`, which
will only run one query (per server) that returns true.
NOTE NOTE NOTE: the above is not 100% thread-safe. It assumes that there is
exactly one user sending requests trying to be the first. If you're worried
about somebody racing to sign up for that first account, you'll want to
come up with a more secure approach.
--Matt Jones
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