Avoid query from calculations on contradictory relation

Previously calculations would make a query even when passed a
contradiction, like `User.where(id: []).count`. This commit optimzes
that to avoid an unnecessary query to the database.

Co-authored-by: John Hawthorn <jhawthorn@github.com>
Co-authored-by: Daniel Colson <composerinteralia@github.com>

activerecord/CHANGELOG.md

@@ -1,3 +1,14 @@
+*   Avoid queries when performing calculations on contradictory relations.
+
+    Previously calculations would make a query even when passed a
+    contradiction, such as `User.where(id: []).count`. We no longer perform a
+    query in that scenario.
+
+    This applies to the following calculations: `count`, `sum`, `average`,
+    `minimum` and `maximum`
+
+    *Luan Vieira, John Hawthorn and Daniel Colson*
+
 *   Allow using aliased attributes with `insert_all`/`upsert_all`.
 
     ```ruby

activerecord/lib/active_record/relation/calculations.rb

@@ -342,6 +342,7 @@ module ActiveRecord
           # Shortcut when limit is zero.
           return 0 if limit_value == 0
 
+          relation = self
           query_builder = build_count_subquery(spawn, column_name, distinct)
         else
           # PostgreSQL doesn't like ORDER BY when there are no GROUP BY
@@ -356,8 +357,12 @@ module ActiveRecord
           query_builder = relation.arel
         end
 
-        query_result = skip_query_cache_if_necessary do
+        query_result = if relation.where_clause.contradiction?
-          @klass.connection.select_all(query_builder, "#{@klass.name} #{operation.capitalize}", async: @async)
+          ActiveRecord::Result.empty
+        else
+          skip_query_cache_if_necessary do
+            @klass.connection.select_all(query_builder, "#{@klass.name} #{operation.capitalize}", async: @async)
+          end
         end
 
         query_result.then do |result|

activerecord/test/cases/calculations_test.rb

@@ -717,6 +717,36 @@ class CalculationsTest < ActiveRecord::TestCase
         Account.where("credit_limit > 50").from("accounts").maximum(:credit_limit)
   end
 
+  def test_no_queries_for_empty_relation_on_count
+    assert_queries(0) do
+      assert_equal 0, Post.where(id: []).count
+    end
+  end
+
+  def test_no_queries_for_empty_relation_on_sum
+    assert_queries(0) do
+      assert_equal 0, Post.where(id: []).sum(:tags_count)
+    end
+  end
+
+  def test_no_queries_for_empty_relation_on_average
+    assert_queries(0) do
+      assert_nil Post.where(id: []).average(:tags_count)
+    end
+  end
+
+  def test_no_queries_for_empty_relation_on_minimum
+    assert_queries(0) do
+      assert_nil Account.where(id: []).minimum(:id)
+    end
+  end
+
+  def test_no_queries_for_empty_relation_on_maximum
+    assert_queries(0) do
+      assert_nil Account.where(id: []).maximum(:id)
+    end
+  end
+
   def test_maximum_with_not_auto_table_name_prefix_if_column_included
     Company.create!(name: "test", contracts: [Contract.new(developer_id: 7)])