Perform two-step Routable lookup by path

In order to lookup a Project or Namespace by path, we prefer an exact
match (case-sensitive) but in absence of that, we'd also take a
case-insensitive match.

The case-insensitive matching with preference for the exact match is a
bit more involved in SQL as the exact lookup. Yet, the majority of cases
will be an exact match. The thinking here is that we can optimize the
lookup by performing an exact match first and only if there is no
result, we perform the case-insensitive lookup.

Data for GitLab.com:
* We have about 15M records in routes table
* About 2,500 routes exist where there's more than one record
  with the same `lower(path)`

It is possible for a user to craft requests that would always trigger
the 2-step search (e.g. we have a route for `/foo/bar`, the request is
always for `/FOO/bar`). In this case, the change at hand is not
beneficial as it would run an additional query.

However, based on the data, it is highly likely that the vast majority
of requests can be satisfied with an exact match only.

The context for this change is
https://gitlab.com/gitlab-org/gitlab-ce/issues/64590#note_208156463.

app/models/concerns/routable.rb

@@ -33,8 +33,15 @@ module Routable
     #
     # Returns a single object, or nil.
     def find_by_full_path(path, follow_redirects: false)
+      if Feature.enabled?(:routable_two_step_lookup)
+        # Case sensitive match first (it's cheaper and the usual case)
+        # If we didn't have an exact match, we perform a case insensitive search
+        found = joins(:route).find_by(routes: { path: path }) || where_full_path_in([path]).take
+      else
         order_sql = Arel.sql("(CASE WHEN routes.path = #{connection.quote(path)} THEN 0 ELSE 1 END)")
         found = where_full_path_in([path]).reorder(order_sql).take
+      end
+
       return found if found
 
       if follow_redirects

spec/models/concerns/routable_spec.rb

@@ -58,7 +58,7 @@ describe Group, 'Routable' do
     end
   end
 
-  describe '.find_by_full_path' do
+  shared_examples_for '.find_by_full_path' do
     let!(:nested_group) { create(:group, parent: group) }
 
     context 'without any redirect routes' do
@@ -110,6 +110,24 @@ describe Group, 'Routable' do
     end
   end
 
+  describe '.find_by_full_path' do
+    context 'with routable_two_step_lookup feature' do
+      before do
+        stub_feature_flags(routable_two_step_lookup: true)
+      end
+
+      it_behaves_like '.find_by_full_path'
+    end
+
+    context 'without routable_two_step_lookup feature' do
+      before do
+        stub_feature_flags(routable_two_step_lookup: false)
+      end
+
+      it_behaves_like '.find_by_full_path'
+    end
+  end
+
   describe '.where_full_path_in' do
     context 'without any paths' do
       it 'returns an empty relation' do