add factory methods for empty alias trackers

If we know the alias tracker is empty, we can create one that doesn't
use a hash with default block for counting.

ActiveRecord::Base.establish_connection(adapter: 'sqlite3',
                                        database: ':memory:')

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
    t.integer :comments_count
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base; has_many :comments; end
class Comment < ActiveRecord::Base; belongs_to :post, counter_cache: true; end
10.times { Comment.create!(post: Post.create!) }

record = Post.first
association_name = :comments

Benchmark.ips do |x|
  reflection = record.class.reflect_on_association(association_name)
  association = reflection.association_class.new(record, reflection)
  x.report('assoc') do
    reflection.association_class.new(record, reflection)
  end
  x.report('reader') do
    association.reader;nil
  end
  x.report('combined') do
    reflection.association_class.new(record, reflection).reader;nil
  end
end

[aaron@higgins rails (tracker)]$ TEST=ips bundle exec ruby ../1bb5456b5e035343df9d/gistfile1.rb
-- create_table(:posts, {:force=>true})
   -> 0.0062s
-- create_table(:comments, {:force=>true})
   -> 0.0003s
Calculating -------------------------------------
               assoc       833 i/100ms
              reader     28703 i/100ms
            combined       839 i/100ms
-------------------------------------------------
               assoc     9010.3 (±3.8%) i/s -      44982 in   5.000022s
              reader  3214523.4 (±5.5%) i/s -   16016274 in   5.001136s
            combined     8841.0 (±5.8%) i/s -      44467 in   5.049269s

[aaron@higgins rails (tracker)]$ TEST=ips bundle exec ruby ../1bb5456b5e035343df9d/gistfile1.rb
-- create_table(:posts, {:force=>true})
   -> 0.0060s
-- create_table(:comments, {:force=>true})
   -> 0.0003s
Calculating -------------------------------------
               assoc       888 i/100ms
              reader     29217 i/100ms
            combined       900 i/100ms
-------------------------------------------------
               assoc     9674.3 (±3.3%) i/s -      48840 in   5.054022s
              reader  2988474.8 (±6.9%) i/s -   14842236 in   4.998230s
            combined     9674.0 (±3.1%) i/s -      48600 in   5.028694s

1 files changed, 36 insertions, 21 deletions

diff --git a/activerecord/lib/active_record/associations/alias_tracker.rb b/activerecord/lib/active_record/associations/alias_tracker.rb
index ea8e1f5054..85109aee6c 100644
--- a/activerecord/lib/active_record/associations/alias_tracker.rb
+++ b/activerecord/lib/active_record/associations/alias_tracker.rb
@@ -7,10 +7,43 @@ module ActiveRecord
     class AliasTracker # :nodoc:
       attr_reader :aliases, :connection
 
+      def self.empty(connection)
+        new connection, Hash.new(0)
+      end
+
+      def self.create(connection, table_joins)
+        if table_joins.empty?
+          empty connection
+        else
+          aliases = Hash.new { |h,k|
+            h[k] = initial_count_for(connection, k, table_joins)
+          }
+          new connection, aliases
+        end
+      end
+
+      def self.initial_count_for(connection, name, table_joins)
+        # quoted_name should be downcased as some database adapters (Oracle) return quoted name in uppercase
+        quoted_name = connection.quote_table_name(name).downcase
+
+        counts = table_joins.map do |join|
+          if join.is_a?(Arel::Nodes::StringJoin)
+            # Table names + table aliases
+            join.left.downcase.scan(
+              /join(?:\s+\w+)?\s+(\S+\s+)?#{quoted_name}\son/
+            ).size
+          else
+            join.left.table_name == name ? 1 : 0
+          end
+        end
+
+        counts.sum
+      end
+
       # table_joins is an array of arel joins which might conflict with the aliases we assign here
-      def initialize(connection, table_joins)
-        @aliases = Hash.new { |h,k| h[k] = initial_count_for(k, table_joins) }
-        @connection  = connection
+      def initialize(connection, aliases)
+        @aliases    = aliases
+        @connection = connection
       end
 
       def aliased_table_for(table_name, aliased_name)
@@ -45,24 +78,6 @@ module ActiveRecord
 
       private
 
-        def initial_count_for(name, table_joins)
-          # quoted_name should be downcased as some database adapters (Oracle) return quoted name in uppercase
-          quoted_name = connection.quote_table_name(name).downcase
-
-          counts = table_joins.map do |join|
-            if join.is_a?(Arel::Nodes::StringJoin)
-              # Table names + table aliases
-              join.left.downcase.scan(
-                /join(?:\s+\w+)?\s+(\S+\s+)?#{quoted_name}\son/
-              ).size
-            else
-              join.left.table_name == name ? 1 : 0
-            end
-          end
-
-          counts.sum
-        end
-
         def truncate(name)
           name.slice(0, connection.table_alias_length - 2)
         end