Avoid creating temporary arrays in ActiveRecord::Result#cast_values in order to speed up pluck

2 files changed, 26 insertions, 5 deletions

diff --git a/activerecord/lib/active_record/result.rb b/activerecord/lib/active_record/result.rb
index 11626c8e31..ffef229be2 100644
--- a/activerecord/lib/active_record/result.rb
+++ b/activerecord/lib/active_record/result.rb
@@ -97,12 +97,21 @@ module ActiveRecord
     end
 
     def cast_values(type_overrides = {}) # :nodoc:
-      types = columns.map { |name| column_type(name, type_overrides) }
-      result = rows.map do |values|
-        types.zip(values).map { |type, value| type.deserialize(value) }
-      end
+      if columns.one?
+        # Separated to avoid allocating an array per row
+
+        type = column_type(columns.first, type_overrides)
 
-      columns.one? ? result.map!(&:first) : result
+        rows.map do |(value)|
+          type.deserialize(value)
+        end
+      else
+        types = columns.map { |name| column_type(name, type_overrides) }
+
+        rows.map do |values|
+          Array.new(values.size) { |i| types[i].deserialize(values[i]) }
+        end
+      end
     end
 
     def initialize_copy(other)
diff --git a/activerecord/test/cases/calculations_test.rb b/activerecord/test/cases/calculations_test.rb
index 6ceac59eec..5c9ed42173 100644
--- a/activerecord/test/cases/calculations_test.rb
+++ b/activerecord/test/cases/calculations_test.rb
@@ -642,6 +642,18 @@ class CalculationsTest < ActiveRecord::TestCase
     assert_equal [ topic.written_on ], relation.pluck(:written_on)
   end
 
+  def test_pluck_with_type_cast_does_not_corrupt_the_query_cache
+    topic = topics(:first)
+    relation = Topic.where(id: topic.id)
+    assert_queries 1 do
+      Topic.cache do
+        kind = relation.select(:written_on).load.first.read_attribute_before_type_cast(:written_on).class
+        relation.pluck(:written_on)
+        assert_kind_of kind, relation.select(:written_on).load.first.read_attribute_before_type_cast(:written_on)
+      end
+    end
+  end
+
   def test_pluck_and_distinct
     assert_equal [50, 53, 55, 60], Account.order(:credit_limit).distinct.pluck(:credit_limit)
   end