You might be tempted to use a partial index in Postgresql for a common scope you have.

An example of a migration looks like this is:

class AddPartialIndexToFoo < ActiveRecord::Migration
  add_index(:foo, :active, where: 'active IS true')
end

The scope could look like:

class Foo ActiveRecord::Base
  scope :active, -> { where(active: true) }
end

The partial index will not evolve with any code changes. A feature request might change the active scope to include another attribute. Once that scope is changed, the index is worthless and it’s hard to remember that it must be changed.

This bit us hard recently and it was hard to pin down because the query timed out only some times and on a variety of pages where the scope was used.

If you have a default scope, it would probably make matters even worse.

Because partial indexes can’t keep up with scopes, I think they are generally a bad idea, like default scopes.